High Power 2-stage - How long coupler?

[Sooner Boomer]

I'm looking at building a high power two stage rocket. Probably a 54mm booster to 1" or 29mm sustainer (maybe 38mm). How long should the coupler be? Ratio by calibers?

 

[cerving]

I usually make mine about 1C. It has to be long enough to keep the sustainer straight.

 

[Titan II]

I use at least 1C on each side.....usually more. My 4" to 38mm has 4" on the booster and 2" on the sustainer.