As of the last question (posted in a message #25): Forms already has that functionality - you just have to learn how to use it.

• enter query mode
• go to "empage" item
• type ":a" in there (without double quotes, simply colon (:) + a)
• press <Execute query>
• a new window will open
• in there, enter
  Code:

  :a between 21 and 50

• push the <OK> button

Otherwise, you'd have to create two items (let's call them "empage_from" and "empage_to") and dynamically create block's WHERE condition using SET_BLOCK_PROPERTY with its DEFAULT_WHERE (or ONETIME_WHERE) properties.

From my point of view, I'd teach my users to use what's offered instead of reinventing the wheel.