Timing when re-flashing

I build and tune marine engines. A good majority of what my customers ask for is to turn up their stock mercury engine. These engines use the mefi ECUs. With this setup I'm not able to live tune. Most of the spark advice tables that are loaded from the factory are set up to act like a distributor with a centrifugal advance. Are there any tricks to estimate the correct advance with out having to wear the engine out on the dyno?

P.S. My dyno is the new Land & Sea Dyno that is capable of handling 3000HP. It is a water brake engine dyno.

The dyno really is the best option for optimising spark timing however if these engines are essentially a maranised LS or older small block chev then you're almost certainly going to be knock limited on pump gas and you can use the knock retard data from a scan to do a pretty good job of optimising spark.