A refundable credit would provide the credit regardless of burden (i.e. it will create a refund for every dollar it credits you beyond your burden).
I think that all PR are important and useful, even fixing a sincgle char in a typo of the docs, but if the incentive is high enought people would send unuseful PR,like fixinf
for (i=0; i<10; i++)
for (counter=0; counter<10; counter++)