You can figure out the Peukert coefficient for your specific batteries by experimentation if you have a way to tell the state of charge for your batteries. But you can only obtain an accurate state of charge with a flooded cell battery by measuring the electrolyte gravity. For all others it is essentially impossible to know the state of charge of the batteries, at in between fully charged or fully discharged.

So, assuming that you have flooded cells, start with an assumed coefficient, drain the batteries down to about half as shown on the meter. Measure the real state of charge and then adjust the coefficient accordingly.

But the Peukert coefficient is an approximation and the rate of discharge can throw it off a lot. I would just stick with what the battery monitor manufacturer says and leave it at that.

And no, combining start and house batteries will have no effect as long as your shunt is on the negative terminal of the house batteries. The load or the charge will be shared by each battery bank while combined but what comes in or out of the house bank is reflected in what the shunt indicates.

David