I'm covering up to 11x14" - and with the arrival of my new jobo that can do 16x20" I'm shopping for holders! And there's always those pesky Collodion chems to find, sky and skill is the limit to size there...

If I look through the lens at an angle using my eye to simulate the extremes of the film plane I can already see the aperture progressively turning into a sort of 'truncated oval' when fully open (what is the correct term?) - If I use a cardboard cut out hole to simulate a Packard I can see it make this truncated oval even more truncated, so I'm just losing a little at the edges which is to be expected. When I stop down I get a complete circle across the full 'frame' but at the extremes the cardboard cut out when it affects the aperture visually now cuts into it a lot faster (than simply the rear element barrel or lens hood does on the other side)

Obvious stuff I guess ...

My question is more how far I should hold my cardboard cut out from the lens to simulate the shutter ? Which is the same question as how do they mount ?