diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index 1f87cfc..4acb234 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -132,4 +132,10 @@ module ApplicationHelper
   def disabled_attributes(disabled)
     disabled ? {disabled: true, aria: {disabled: true}, tabindex: -1} : {}
   end
+
+  def number_attributes(type)
+    step = BigDecimal(10).power(-type.scale)
+    max = BigDecimal(10).power(type.precision - type.scale) - step
+    {min: -max, max: max, step: step}
+  end
 end
diff --git a/app/views/units/_form.html.erb b/app/views/units/_form.html.erb
index f59b3a8..f02f404 100644
--- a/app/views/units/_form.html.erb
+++ b/app/views/units/_form.html.erb
@@ -14,8 +14,8 @@
       <% unless @unit.base.nil? %>
         <%= form.hidden_field :base_id, form: :unit_form %>
         <%= form.number_field :multiplier, form: :unit_form, required: true,
-          step: BigDecimal(10).power(-@unit.class.type_for_attribute(:multiplier).scale),
-          size: 10, autocomplete: "off" %>
+          size: 10, autocomplete: "off",
+          **number_attributes(@unit.class.type_for_attribute(:multiplier)) %>
       <% end %>
     </td>
 
diff --git a/test/application_system_test_case.rb b/test/application_system_test_case.rb
index 9372b8e..2f70892 100644
--- a/test/application_system_test_case.rb
+++ b/test/application_system_test_case.rb
@@ -34,6 +34,12 @@ class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
   #  assert_raises(Selenium::WebDriver::Error::StaleElementReferenceError) { element.tag_name }
   #end
 
+  # HTML does not allow [disabled] attribute on <a> tag, so it's not possible to
+  # easily find them using e.g. :link selector
+  #Capybara.add_selector(:disabled_link) do
+  #  label "<a> tag with [disabled] attribute"
+  #end
+
   test "click disabled link" do
     # Link should be unclickable
     # assert_raises(Selenium::WebDriver::Error::ElementClickInterceptedError) do
diff --git a/test/system/units_test.rb b/test/system/units_test.rb
index 0a92352..4c51a22 100644
--- a/test/system/units_test.rb
+++ b/test/system/units_test.rb
@@ -23,18 +23,24 @@ class UnitsTest < ApplicationSystemTestCase
     end
   end
 
-  # TODO: check if Add buton is properly disabled/enabled
-  # TODO: extend with add subunit
   test "add unit" do
-    click_on t('units.index.add_unit')
+    add_link = all(:link, exact_text: ADD_UNIT_LABELS.sample).sample
+    add_link.click
+    assert_equal 'disabled', add_link[:disabled]
 
     within 'tbody > tr:has(input[type=text], textarea)' do
       assert_selector ':focus'
-      maxlength = all(:fillable_field).to_h { |f| [f[:name], f[:maxlength].to_i || 1000] }
+
+      maxlength = all(:fillable_field).to_h { |f| [f[:name], f[:maxlength].to_i || 2**16] }
+
       fill_in 'unit[symbol]',
         with: SecureRandom.random_symbol(rand([1..15, 15..maxlength['unit[symbol]']].sample))
       fill_in 'unit[description]',
         with: [nil, SecureRandom.alphanumeric(rand(1..maxlength['unit[description]']))].sample
+      within :field, 'unit[multiplier]' do |field|
+        fill_in with: random_number(field[:max], field[:step])
+      end if add_link[:text] != t('units.index.add_unit')
+
       assert_difference ->{ Unit.count }, 1 do
         click_on t('helpers.submit.create')
       end
@@ -44,7 +50,9 @@ class UnitsTest < ApplicationSystemTestCase
       assert_no_selector :fillable_field
       assert_selector 'tr', count: @user.units.count
     end
-    assert_selector '.flash.notice', text: /^#{t('units.create.success', unit: @user.units.last)}/
+    assert_no_selector :element, :a, 'disabled': 'disabled',
+      exact_text: Regexp.union(ADD_UNIT_LABELS)
+    assert_selector '.flash.notice', text: t('units.create.success', unit: @user.units.last.symbol)
   end
 
   # TODO: check proper form/button redisplay and flash messages
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 63bc8df..2bb8f4f 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -32,6 +32,18 @@ class ActiveSupport::TestCase
     "%s@%s.%s" % (1..3).map { SecureRandom.alphanumeric(rand(1..20)) }
   end
 
+  # Assumes: max >= step and step = 1e[-]N, both as strings
+  def random_number(max, step)
+    max.delete!('.')
+    precision = max.length
+    start = rand(precision) + 1
+    d = (rand(max.to_i) + 1) % 10**start
+    length = rand([0, 1..4, 4..precision].sample)
+    d = d.truncate(-start + length)
+    d = 10**(start - length) if d.zero?
+    BigDecimal(step) * d
+  end
+
   def with_last_email
     yield(ActionMailer::Base.deliveries.last)
   end