Summary

At line 49, when it's checked if an interval is lesser than the interval range, the formula is inefficient.

Vulnerability Details

Current formula is the following:

uint256 timePassedBefore_ = startTime_ - payoutStart_;
if ((timePassedBefore_ / interval_) == ((endTime_ - payoutStart_) / interval_))

Computing the operation doesn't even require the "payoutStart" varaible, as it would be equivalent with testing (endTime - startTime) / interval_ == 0.
A small optimization can be further performed by storing (endTime - startTime) in an uint128 variable (proposing name timeDelta) and using it on line 53. By performing this substitution, readability is also improved.

Impact

Extra gas cost for operation

Tools Used

Manual inspection

Recommendations

Rewrite formula as suggested:
if (((endTime - startTime) / interval) == 0)

And for further optimization & readability:

uint128 timeDelta = endTime - startTime
if((timeDelta / interval) == 0) {
...
return (intervalFullReward_ * timeDelta) / interval_