Debt repayment and investment strategy: Pay down more debt when P/E for stocks is high, buy more stocks when P/E is low.

I am looking for a formulaic way to emphasize either debt repayment or equity purchases based upon the price of the S&P 500 or other index (based on P/E) and the interest rate of the debt. To clarify, I would like to be able to find a formula that says based on the 3% interest on your debt, and the 20 P/E ratio of the S&P 500, you should put xx% toward debt and xx% toward in equities.

During market slumps, I'll often put off debt repayment in order to purchase more in equities. I think that this is a good strategy, but I would like to remove the emotion from these decisions. Does anybody follow a strategy like this? Are there any formulas like this online or in books?

I know that PF has a strong pay debt off first bias, but I'm of the mind set that under current interest rates it will be beneficial in the long run to delay debt repayment in favor of investing. I'm not looking to be told to pay off my debts first. They are all very low interest loans and I fully understand the risk of delaying repayment. I already max out 401(k), Roth IRA, and HSA, so I'm looking for advise on what to do without regarding potential tax benefits of investment vehicles.