I'm looking into insulating the attic above a closed-in back porch & the estimate also included adding additional insulation on top of the blown-in in the attic.

What kinds of factors can be used to think about the value of additional insulation?

For reference, I'm in Florida & keep setpoint around 80 degrees F most of the time when home, and 86 when not home. Power bills in the summer are in the $150-$180 USD range.

It's currently R27 and the quote is to add R11 to bring it up to R38. Code here appears to put new construction at R38 as a minimum, but looking at some charts - it looks like I might have already hit diminishing returns?

This chart I found on "Energy Vanguard" seems to suggest that going from 27 to 38 isn't much of a difference.

Does anyone else have any insight on when those diminishing returns are hit, and if it can make any appreciable difference in power bills? The house itself is comfortable enough, without large swings in temperature.