Some interesting thoughts here. I would generally disagree with including the interest portion of student loan payments in a savings rate, since they are an expense in every sense. Principal makes sense, because it does have a positive impact on net worth, but the interest you pay doesn’t. Obviously in the future, when the debt is paid off, you can put the full amount towards savings, but since you currently aren’t, and the full payment isn’t currently affecting your net worth, it doesn’t make sense to include the full amount.
I would also argue that the logic you apply to justify including the interest portion could also be used to make an argument to use after-tax income, rather than gross, to calculate savings rate. Taxes are easy to reduce (to a point) by contributing to tax-advantaged accounts, interest on student loans is easy to reduce by paying down your debt. There are also other items that get deducted from your gross income that you can’t reduce (CPP & EI in Canada, what I believe are called FICA taxes in the US) which are unavoidable and not affecting your standard of living, not to mention something that you likely won’t be paying in retirement (just like taxes, in all likelihood). Aso, in the way that if you needed student loans to get your job, if your job is responsible for your high taxes, they are not optional, lifestyle inflation-type expenses that you need to cut back on (although I would argue you should do as much as possible to minimize them).
My method seems to be slightly different that most people’s here. I calculate my net worth each month and divide the change in my net worth by my net income to get my savings rate. If I get something like a tax refund or a meal allowance from my job (they always go straight into savings), I add it to my net income. I think in the US it would also make sense to add back your employer’s match (in Canada, it’s included in income and then deducted again, basically just an in-and-out for a net impact of nil). This method might change at some point, because I’m still at the point where market returns have basically no impact compared to my actual contributions, but for now it works. I feel that it captures everything I need it to: My own saving contributions, my employer match, my debt reduction, and any income (i.e. dividends) from investments that gets reinvested rather than spent. As with everything else personal finance-related, lots of ways to do the same thing, it’s just a matter of finding what works best for and makes the most sense to you.