I don't know if I understood your question correctly but you just choose an "auto" level in the level settings of the engine. Then Pico<chess will adopt your and the engines elo accordingly after the game.
Note that not all engines do support auto elo levels (but stockfish for example does.)
If you want to start with a specific new own elo just reset the value to your initial elo rating in the picochess.ini file (tag "pgn-elo", eg. pgn-elo = 1200 and set tag "rating-deviation" to initial rating-deviation = 350). Maybe in your picochess.ini the tag "pgn-elo" is not set at all, then set it to your guessed elo value.
Dirk