We used to have a £10rb in Bournemouth that was 500 chips starting and the same for a rebuy. You got a treat for the add on though at 2k iirc

Used to be good prizepools though and absolute carnage!!
It's finding the balance, so that the players can't just sit there and treat it like a f/o and have to get involved. On the flipside to that, you have to accept that making it punty will result in losing some players.
I'd say the best way to do things would be to have a bigger add on instead of just a double, that will encourage more people to stay in until that point I think and it won't make much difference in the grand scheme of things anyway. People find it just as easy to flick in 50bb with a pair as they do 20......
Rambled on a bit, but I'd say 1k chips and a 5k addon would be good. Should ensure that people punt enough unavoidably and they will stay in to get the 5k after 2 levels.
Also agree with making the r/b levels 40 mins, think that was what ours was and it still nearly always finished by 3 from an 8pm start with about 50 runners.