How to use existing S/L depreciation figure in 2020 tax forms?

If you enter the asset in the program and include the prior year depreciation taken then the program will automatically compute the correct current year depreciation figure. 

You don't just use last year's figure per-se. There are many factors that can change the depreciation you're allowed to take in any given tax year. For example, personal use of the property, partial loss of or damage to an asset, and many more possibilities. If you just enter the data in the program that the program asks for, then it will figure the correct depreciation for you, based on the data you enter.