Hi techs

I am doing a quote for job.

25 year old home
Bungalow - 2 furnaces with seperate duct systems.

House is presently zoned by one furnace covers half of main floor and basement and the the other furnace covers the other half.

I am going to propose 1 furnace with a 2 zoneing system.

My question:

In sizing the furnace, is a standard block load calculation acceptable in establishing the furnace size?
I look forward to hearing input and good advice from techs
who have installed zoning systems.
Also preferred method of air byspass when only one zone is calling. The basement is unfinished.

Thanks you
Servicepro