There is already a word for that concept

Before

Performance targets include a threshold goal (performance below which would result in no payout being made), target goal, and maximum goal (performance above which no additional payout is earned). Achievement of levels between goals is typically equal to a percentage of base salary or number of shares that is on a straight-line basis between the two goals but can be on any basis established by the Committee.

Before, with commentary

Performance targets include a threshold goal (performance below which would result in no payout being made), target goal, and maximum goal (performance above which no additional payout is earned). Achievement of levels between goals is typically equal to a percentage of base salary or number of shares that is on a straight-line basis between the two goals [There is a word to describe this idea: proportionate. Why not use it?] but can be on any basis established by the Committee.

After

Performance targets include a threshold goal (performance below which would result in no payout), target goal, and maximum goal (performance above which would result in no additional payout). Payouts typically change proportionately for achievement at levels between goals, but the Committee can opt to make payouts at different percentages.