So I have started building the 29x2.1 wheel set to replace the 26x4.0 on the VIr Fortis on 150/197 hubs. I have also go the option of a 5th wheel 29x2.1 on a standard non fat dynamo hub. If I get this one as well, I will need a new set of a 29 suspension forks.

What formula should you use when replacing rigid with suspension forks to keep the geometry the same?

Many thanks

From my recent asking:
You need to get a suspension fork about the same dropout to crown length with the fork in its sagged position.
Just did that myself and the bike rides fine.
The trail is different on the suspension fork but I can’t tell the difference.

Ta i’ll Get the tape measure out just in case.

The sagged sus fork length imo gives a fork that can be a bit long for ideal all-round handling. It works OK, plenty of people / brands doing it. My take on it is that passive (seated) sag is less than the average ride height of a working sus fork, ie moving over bumps or compressed though corners etc. 20-25% seated sag vs 30-40% (modal?) average compression. So, 35% of travel taken off the sus fork length works pretty well.
Gives you a geometry only 1/2 degree steeper on average than the std sag guide so setting up seat angle / saddle position is no bother and you get (what feels to me) closer to the bike's handling with a sus fork.

Going the other way round and just yesterday did the calcs from a Krampus to a Fox 34 with 130 mm travel. Will reduce the travel to 110 mm, this will give me a 32 mm longer fork when not sagged. Sagged I go by 30 % which is equal to 33 mm. Bingo!
I take sag measurements in the position in which I want the bike to perform as set up.

As it's for offroad travel, we intend to run the fork with all tokens and little pressure to give a supple feel mid stroke and early progression thanks to the smaller chamber. Damping is much generally weaker than to what I ride on my trail bikes.
There's a lot to play with in there...