Fixed cost remains constant if expressed on a unit basis?

Basically when you refer to fixed costs you are talking about expenses that remain "constant" over a longer period of time, such as a lease, and other "overhead" obligations. These fixed costs are defined as a percentage of total gross so in that case it is a percentage point which is the same as a unit. Fixed costs don't actually remain constant, but in order to run a business you need a business plan and you need to plug in the plan numbers for 12 months, based on payments that remain consistant and by using last years numbers to create a monthly average number for things like untilities. If you don't have planned numbers that are pretty solid you need to make adjustments throughout the year to keep them as close to plan as possible. Otherwise your overhead can eat up the money you need for inventory, which is where the profit margin comes from to pay for everything and give you an ROI that is viable.