I've got a question for all you brainiacs.
While putting some cleats on the Bonts I picked up over the summer, I encounter a math word-problem.

In attempting to transfer coordinates from my older Mavics to my new Bonts, here's what I encountered.
Existing setup: Mavic Pro Road shoes. USA:8.5 EURO:42.5 Time RXS Cleats
New Setup: Bont A3 shoes. USA:8.5 EURO:42.5 Time RXS Cleats
Both shoes fit my feet well, the Bonts a bit better than the Mavics, both are correctly sized.

Some tape-measure work tells me that the Mavic marked 'Centerline' of the shoe is 2cm (exactly) forward of the Bont 'Centerline' on the Bont Grid.

My previous set of RXS cleats was mounted about 4mm behind what Mavic has marked as the "0" mid-line of the shoe. This left the cleats say... 75% slid back on the track of the cleats. Cleat centerline = Mavic centerline -4mm. (Or Cleat centerline 4mm behind marked shoe centerline)

On Bonts: Using identical Time RXS cleats on the Bonts - sliding the cleat 100% of the way forward leaves them at Bont Grid Centerline +12mm.

Thus, the cleat holes are
Mavic: "Centermark" - 4mm = Existing Cleat Position.
Bont: "Mavic Centerline" -20mm for Bont Grid position + 12mm (max forward cleat position) +4mm (adjustment for cleat position on Mavics) = -4mm cleat position compared to Mavics, closest possible position to existing position.

Google tells me that Mavic's shoes are drilled waaaay far forward, and it would seem that yes, this is borne out by the fact that I can't get my cleats to line up, even sliding them all the way forward.

Am I thinking this out incorrectly? Is there something I could do differently?

1) Put old shoes on

2) mark center of ball of foot

3) mark cleat for same

4) put on new shoes

5) mark center of ball of foot

6) line up mark on cleat

If ya can't do that, then you have a problem that may need to be McGyver'd.

Also be aware the stack heights between the two will probably be pretty different.

After reading the word-problem about 1.8 times, I recommend the following:
1) Listen to RacerEx's comment above.
2) Seriously consider going by feel as well. I have some back story on this.

A little more than a year ago, I purchased new shoes; last shoe purchase before that one was in the early 1990s (Time Carbon Racing -- oh, how I loved them). I tried "translating" the cleat position over using various measurements and such. Long story short, it wasn't working for me. In the end, I went by feel, and, at present, I am a happy camper. I have "objective" powermeter data (as well as "feel" data) that somewhat confirms that I have things set up reasonably well. My key was focusing on how my foot felt and where it was connecting during the pedal stroke with the old shoes, and then trying to get that same feel with the new shoes. It took some time and focus.

I understand that the "numbers" don't look good. Try an approach where you ignore the numbers. Go with feel. (Kinda Luke Skywalkerish now that I thing about.) Then, if that doesn't work well enough, uh, well, er, we'll figure something out.

Marking the ball of foot is pretty handy and has worked for me over the years but with the last pair of shoes I got something just didn't feel right so I did it by feel until I got it right. It took some trial and error but eventually my feet were happy.

Another trick is to put some masking tape on the outside of both pairs of shoes, and use a Sharpie to mark the center of your big and little toes' 0th knuckles and use the imaginary line between them to position your cleat the same way on both shoes. Once you get it all dialed in, the tape leaves nothing behind.

Modern adaption of the "old school" method:

Way back when in quill pedal days you would ride your bike for a bit without cleats. The rear "edge" of the quill pedal would leave a mark in the bottom of your shoe and then a cobbler (or somebody you trusted) would nail the cleat to the soles with the "slot" lined up with the mark.

When I transitioned to "clipless" this idea wouldn't work, so I came up with a reasonable facsimile. I loosely attached the cleats to the shoes and hopped on a trainer for a ride. When it felt good I stopped and had my wife mark the cleat position with a Sharpie. Twisting out moved the cleats, of course, but now I had marks to do alignments before tightening them up. Worked for me.

My first time ever using this expression: Like!