Enable Range Calculation, calculating the minimum and maximum value dependent on other values in your Gravity Form using Merge Tags.
Enable Minimum / Maximum Value Calculation
You can select ‘Enable Minimum Value Calculation’. You see that under ‘Range’ the ‘min’ input is greyed out, since we’re going to use a calculation to determine what the minimum value of the range should be. You can use Merge Tags (like you’re used to) to determine the minimum value and set a Minimum Value Instruction. It works the same way with Maximum Value Calculation.