I decided to put a quick drawing and spreadsheet together to help others with deciding a lower playfield design option.
The scenario is using the standard available Bally / Williams return lane guides or the Data East / Sega / Stern return lane guides only.
Please those with custom pin building experience add to this, correct me if needed
But if sticking to standard off the shelf parts the lane guides and flipper spacing is what determines a 1 of 6 possible layouts. Flipper spacing choice is between 6 3/4", 6 7/8", 7" and 7 1/8" spacing with the middle 2 being the most common.
Once you decide what you want with flipper spacing and your preferred return lane guides you can quickly see that section "L" is the critical factor to be adjusted (it's not much but enough to scrap your prototype playfield). And for example option 6 you would have to rely on the divider between the rhs outlane and the plunger lane to be a section of 1.5mm SS. You can tweak the return lanes, outlines and sling shot design a little to try and achieve a certain design but if you want to add a extra return lane on one of the sides Section "F" & "J" would have to be custom return wire lane guides and not the moulded plastic ones readily available. Also I doubt you could have a comfortable spaced lower 1/3 layout with 7" flipper spacing or more with Bally / Williams return lane guides.
"H" isn't a critical figure - more a result of other figures to aid in determining the width of section "L"
Sections - D & N as well as A & C would hardly ever change and would remain a constant measurement.
I hope it helps and my apologies for referencing millimetres but that is my preference for the fine details or you could simply divide by 25.4 to get the imperial size. And this is merely some info to help those wanting to design a game to be able to quickly determine spacing of the lower third of their game.
Also please double check dimensions of parts purchased against the ones I have provided to ensure there is no variances out there from different suppliers.