How to calculate RMSE for a non linear model?

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I am doing a project in which I need characteristics for signal strength to distance. I have done measurements and made a curve fit of mean values. For a log-plot the model becomes linear and an RMSE value can be calculated.

Log Distance Model = -0.02872*x - 0.2217

RMSE = 0.0527

RMSE lines = -0.02872*x - 0.2217 $\pm$RMSE

Log Distance Model

However, when transforming the Log Distance Model to a Non-Log Distance Model, to get the real distances in meters. The upper and lower RMSE boundaries are not equal in magnitude ( $|$+RMSE$|$ $\neq$ $|$-RMSE$|$ ).

Non-Log Distance Model = 10^(Log Distance Model).

Non-Log RMSE lines = 10^(RMSE lines)

Non-Log Distance Model

Am I doing it wrong? If so, how should I get RMSE values for the Non-Log Distance Model?

If it is correct, how is it logical that the $|$+RMSE$|$ $\neq$ $|$-RMSE$|$ ?

Best regards