If you know what you paid in premiums, deduct that total from the amount you received as "cash-out" (Box 1) and put the resulting figure in Box 2a.
This post addresses the same issue you are facing and makes a couple of suggestions - 1099R for cash out of life insurance
In my opinion, @DavidD66 's suggestion is the best approach. It results in an accurate tax calculation, which is what the IRS is most interested in achieving..
His commentary was: "your insurance company should have determined the taxable amount and included it in Box 2a. The only way I can figure out how to report the 1099-R, and get the correct taxable amount to flow to your Form 1040 is for to calculate it and enter it in box 2a of your 1099-R entry. This will result in the correct taxable amount flowing to your Form 1040, line 4b."
