For what it's worth, I've been using a combination of things to set my gears. Time will tell if I've been successful.
Firstly I set FD to as high as it'll go and Top Speed to as low as it'll go. I then examine each gear and note the min/max. I have a simple spreadsheet that I use for this.
Column A lists the gears.
Column B lists (what I call) the 'Multiplier'.
Columns C & D show the min and max ratio for each gear.
Column E is derived from B, C & D "=((D-C)/100)*B+C"
I also have three further columns that have formula derived from Scaff's TuningGuidePart2.pdf. Whilst these three columns are prone to inaccuracy, due to trying to find/guess tyre sizes for any given car, they give you an idea of the speeds you might achieve in any given gear. Combined with using the results of Column E to make a simple chart, I believe you can make a good stab at setting gears.
Finally, the 'Multiplier' is a simple figure between 1 and 100, a percentage. I've found that spreading gears exactly evenly can cause problems with the lower gears. So, for instance, I have an Evo that has the following spread of ratios;
1st 50.0%
2nd 25.0%
3rd 20.0%
4th 20.0%
5th 25.0%
6th 50.0%
This sets my ratios as;
1st 2.989
2nd 2.027
3rd 1.541
4th 1.239
5th 1.047
6th 0.869
With FD set to 4.150, I'm getting the following speeds from each gear (approx);
1st 41.9mph
2nd 61.8mph
3rd 81.3mph
4th 101.2mph
5th 119.7mph
6th 144.3mph
I'm sure there are more scientific ways of doing it, but to be honest, it suits me as I can't afford to get bogged down in the specifics. I just want to pump some numbers in and then change my Multipliers to get ratios that feel good around a track. The above Evo ones are for a car that will spend most of its life off road, so I wanted 2nd -> 5th to be quite close with 1st and 6th a bit longer. Hence my Multipliers.
My Nordschleife LFA has the following spread of ratios;
1st 60.0%
2nd 35.0%
3rd 25.0%
4th 25.0%
5th 35.0%
6th 60.0%
I need more coffee, sorry if the above makes no sense, I hadn't planned on writing it if I'm honest. Just kinda vomited it out...
{Cy}