how to calculate "-103.7dbm + -108.2dbm" faster?

how to calculate "-105.6dbm + -108.2dbm" faster?

i know the result is -103.7dbm, but my calculating process is slow.
Does anyone know how to get it by an easy way?

Use a lookup table. You take the difference between the levels as the entry point in the table and add the found value to the larger of the two signal values. It does not take many table entries to find values from 3 dB to 0.1 dB of addition.

When I had to do this frequently (many years ago before portable computers were afordable) I drew a graph of the addition factor.

As a piece of history, the early word processors ran on relay rack size computers and you had a remote dumb terminal. You alternated a line of formatting code with the text that was to be formatted. This was even more cumbersome than writing HTML code for web pages. You had to print out the results on a line printer (that was always on the opposite side of the building from you or even in the next building) to see the errors and revise yet again.

could you tell me where i can get a "lookup table"?

You have to calculate it yourself, but you only calculate it once. You may need several tables for situations where the signals are uncorrelated and for when they are the same signal at different phasees.

Use the same method you now use. Make one signal 0 dBm and the other from 0 dBm down to how many -dBm it takes to get an addition of 0.01 dBm of total power.