could you explain a little bit more what you try to calculate? the example you mentioned would always give 100% (because every unique user must have a "first visit" in a defined date range). But I guess I missed a point...
To get the "first visits" in a certain time range you need to combine 2 cases:
visitors with 1 visit in the desired date range: include this visit
visitors with 2 or more visits in the desired date range: you need to get the first visits by do some stacking of visitor and visits containers (basically very similar as the Tip from @audreysalerno: bit.ly/SalernoTips2018)
if you combine those 2 segments you have all the first visits in a certain date range.