I obtained some Rf values via TLC by adding two or three drops methanolic triethylamine to an eluent mixture of let's say DCM/MeOH = 9:1. I couldn't find an example of how a mixture like that is written down in short in publications. I think that calculating the absolute content of dissolved ammonia wouldn't be suitable.
Describe what you did. The key is make sure your communication allows your experiment to be reproduced.
... Rf 0.34 (10 mL of 9:1 DCM/MeOH + 3 drops of 1.0 M Et3N in MeOH).
If you do all of your TLC with the additional of triethylamine, then you can include a sentence in the general methods about this procedure.
All TLC plates were developed in a 9:1 mixture of DCM and MeOH that was made alkaline by the addition of three drops of 1.0 M Et3N in MeOH for every 10 mL original solution.
The procedure you describe is common, especially in the elution of basic molecules (since silica is slightly acidic). You might find examples of how to state this sort of procedure in the synthesis of amines and alkaloid natural products. Ultimately, in a publication, you should err on the side of thoroughness for your experimental methods. If it's too long, a reviewer of the editor will tell you.