Expressions for Discount

[JTRTech]

I would start here;

Custom Keyboard Codes

As of v4.1.61 we can create Custom Keyboards to use with [?prompt] notation, and a Custom Keyboard Widget is available for Entity Screens. For [?prompt] notation, the syntax is: [?<prompt>;<mask>;<default value>;<flags>;<keycodes>] <prompt> is the question or prompt to display <mask> is a regular expression to define what type of characters (and how many) are allowed <default value> allows for a common value to be pre-entered <flags> lets you define the keyboard type and which buttons …

Although doesn’t mention setting a maximum.
Perhaps it was setting default value of 10 and the mask only allowing single digit…
[?Prompt] expression offers these options [?<prompt>;<mask>;<default value>;<flags>;<keycodes>]

If you made it a two stage flow with the prompt in an execute automation command in value fields you could define all sorts of prompts should a value be entered wrong using constraints on the command value lide [:CommandValue] > 30 on an ask question prompt saying discount only possible up to 30% and then your action to apply discount would have constraint of [:CommandValue] < 30