The head on a cast iron or steel boiler is typically small relative to what it is on the coil in the indirect, but as indirect heat exchangers have improved it's not always the case. But if you assumed the head on the boiler is zero and size the pump for the head on the indirect + plumbing + 25% it'll usually be good enough. If you're cutting it close you can assume the boiler's head to be something like 1-1.5 feet at ~10gpm, (in most cases it's less than 25% of the total.)
The gpm specs for getting the first-hour hot water gallons into the indirect aren't always sufficient to keep the boiler happy. On an old-skool cast iron boiler need enough pump to keep the delta-T on the boiler within spec- shoot for 25F or less unless you know for sure it can take more.
An output of 108KBTU @25F delta-T is a flow of (108000/25F=) ~4320lbs/hr, which is (4320/60=) 72lb/min, which is about (72/8.34=) 8.6gpm. If you sized the circulator to be able to deliver 11gpm+ against the head of just the indirect + plumbing you'll be good, but 10gpm might be cutting it close if it's a low-head indirect. There's little downside to erring the high side on flow- high flow=higher turbulence=better heat transfer efficiency in both the boiler and the indirect. You pay a bit more in electricity, but duty cycles are small. The bigger cost will be in the pump itself.
A primer on the subject lives here: