A couple of questions first. What is the temperature setting on the indirect? The combination of setting that hotter and adding a tempering valve (if you don't have one) would extend the safe amount of water in the system.
How much hot water in one session do you want?
Is the tank on a priority zone? This means that when the indirect calls for heat, it is the only zone that gets it, letting the full output of the boiler work towards keeping the water hot.
How big is your boiler?