My take home pay is around $54,000. Is this the right number to be using for calculating savings rate, or should I be using gross pay?
There is not "right" or "wrong" way to calculate savings rate. There is only your way. Savings rate is simply a broad and imperfect personal finance metric. Windfalls, promotions, job loss and emergency expenses can shift savings rates, often to comical proportions. Ultimately what matters is the absolute $ amount that you save, and the absolute $ amount that you plan to spend per year.
In other words, don't loose sight of the forest ($ savings) for all the trees (% savings).
Personally, when I do check on my savings rate (which isn't often) I use gross pay but I do include the equity portion of my mortgage payments in addition to other contributions. I use gross income because 1) it's more strict and 2) to a large extent I can control how much I pay in taxes each year by utilizing tax-advantaged accounts.
But whatever works for you.
I am pretty sure I'll never save 75% of my take home pay as that leaves $13,500 to live on.
$13.5k would certainly be a very frugal lifestyle, but plenty of people do exactly this, either by choice or necessity. It's quite a bit less than I'd like to live on, but on the flip side you could start from $0 and be FI in 6-8 years
http://earlyretirementextreme.com/