1. Borrow $A at an interest rate of I (I = .078 means 7.8%).
2. Invest it at a Return of R (R = .123 means 12.3%).
3. After N years you cash in your investment, pay the taxes and the interest on the loan and pocket the balance
   ... if there is any balance!
4. Your marginal tax rate is T (T = .45 means 45%).
5. You pay only a fraction f of the taxes (f = .75 means 75%, for Capital Gains).

After N years your $A investment has grown to A(1+R)^N
so your gross gain is A(1+R)^N - A.
You pay taxes on this, namely fT[A(1+R)^N - A]
leaving you with [A(1+R)^N - A](1-fT).
You also have the interest to pay, namely IA per year for N years, amounting to IAN.
You're now left with [A(1+R)^N - A](1-fT) - IAN to spend.

Dividing by the original investment of A we get the gain per dollar of investment, namely:

[(1+R)N - 1](1-fT) - IN

Expressed as a percentage, that means a gain of

{1 + [(1+R)N - 1](1-fT) - IN}(1/N) - 1 per year.

Example:
A = $50,000 borrowed for N = 5 years at I = .07 (7%) and invested at R = .1 (10%) after which you cash in, pay taxes at your marginal tax rate of 45% (T = .45) ... but it's capital gains so you only pay 75% of the taxes (f = .75).
You get (after taxes and paying the interest): [(1+.1)⁵ - 1]{1-(.75)(.45)} - .07(5) = $0.2120 for each dollar invested.
That's an annual (shall we call it a) "Return" of (1+.2120)^(1/5) - 1 = .039 (or 3.9%) ... after taxes.

Here's a spreadsheet y'all kin try.