I have two employers so getting back SS overpmt. When estimating safe harbor estimated tax w/h, can I count SS refundable credit towards total w/h for est taxes?

I wouldn't, unless you are planning to have the same issue in 2022. 

Yes, I am fairly certain based on salary I will get the SS overpayment again in 2022.  Sounds like you're saying if that's the case, then refundable credits (including for SS) would count toward total taxes paid when formulating if there's been an underpayment

The Social Security overpayment is not only based on your salary but it is based on the fact that you had two separate employers both withholding Social Security tax.  

Therefore, you must also have two employers and a salary that exceeds the maximum limit before you should include that credit when calculating your estimated payments for 2022.  Normally, this is an unusual situation when you may have changed jobs during the year, not something that you can count on again.

@Ryo1