JohnCW
JohnCW The way you are using the guitar tuner gives you a visual reference for tuning a unison. But the overall note placement is unlikely to be correct. You will get a better result learning to tune the unison by ear (because you'll be considering the fundamental and all partials that make up the tone). If someone can tune a guitar by ear, they are well on their way to being able to do a reasonable job of tuning a piano unison. At least good enough to get by till the next professional tuning.
Well, I did try to avoid getting into the details with my suggestion...
As for using PianoMeter or some other app that DOES support temperament calculation, IMHO things can get even MORE complicated, because the piano may have drifted up/down a bit since the tuning, not to mention that certain sections might move in different directions!
The method I suggested is the one I use myself, and it generally helps me keep things "tidy" until the next time I do a full tuning - generally at a change over from heating to cooling or vice-versa, and that's when the biggest swings in RH occur. I'm a PiaTune proponent. Hakki has done an amazing job improving it to where it has become an extremely sophisticated and powerful tool that can be used fairly easily in what is analogous to "point and shoot" mode with a cellphone camera. And in case anyone should ask, I have Damp Chaser systems on my grand as well as my upright, and in the winter I add a Venta LW-25 which generally can keep humidity around 40% when things get really cold and a bit higher as the temps go up.