Do you want validation of the input or formatting of the output?

Both would be good but at the moment there is no decimal field. Closest are integer or text.
So for example if you were to add a price to your recipes in the demo, how would you? If you stored them as text then to then add to a basket or add up the cost of several then its not going to be a proper numeric calculation. I guess you could store as text and then cast as decimal but that just feels wrong, and integer is no good as prices have 2 decimal places and if stored in a table as an integer the decimal places will be lost.
Or am I missing the point and its all stored in the tables as varchar and we dont need a decimal custom field?

You can use a text field and set the filter to 'float' (which is the same as decimal) - but it won't format the output with a fixed number of decimal places.

We could add a format option to EasyLayouts to format the output in a particular way so that could also handle currency formatting (with symbols, commas and decimal places).