What I have done in the past is to get a .010 density filter from kodak(the gel one that is 3" square) and take that on top of a film that was not exposed but developed with your other test sheets and use your spot meter on a tripod and read through the .010 filter and blank neg at a diffused light. I use the globe under our kitchen ceiling fan and it has worked every time. Take that reading and write it down. Then take readings of all your other negs and the one that is closest to the original reading will be your .010 above film base plus fog. It will also be your new personal film speed. I have done this several times and it is slick. An old timer showed me it and he called it a poor mans densitometer. Hope I haven't confused the hell out of you!