Hi @kreetelyll,
Thanks for clarifying!
I think you are making the right choice by using the forward reads only, give that you only have a 150bp run.
As for the truncating parameter, 122 sounds very reasonable. Given that the median score in your reads is above 35 even almost all the way through you could probably increase that value to something higher as well. The difference between 122 and say 140 example would be minimal with the former retaining probably a small amount of more reads at the cost of slightly less resolution. I use probably and slightly carefully here because these are both dependent on the data itself. You can always run a few different parameters and compare to see what works best for you. This gives you a good in depth understanding of how parameter setting affets a run. That being said, like I mentioned, you could totally just carry on with your analysis there are much more important challenges coming your way and you don't want to obsess over 1 step 
As for the tool you linked too, I had not heard of FIGARO, but it does look very promising and would make a great q-2 addition since we do get a tremendous amount of questions on the forum regarding how to pick these parameters. Having read their brief pre-print, I would have liked to see them benchmark and validate their output recommendations but that may just come with use. Though unfortunately this will not help your case since you are using forward primers only and FIGARO is built for decision making regarding paired-end results. Thanks for sharing though!