Newbie PPM question

So I am doing my 3rd grow my first 2 were in soil, this one I am doing a coco hempy bucket (chunky perlite res with a 40/60 mix of perlite coco on top). I got a ec pen to measure ppm, and want to feed my seedlings 250ppms of nutes (going lucas formula so its easy just nova flora bloom and some epsom salt). My question revolves around taking the tap water ppm into account.

For example my tap water comes out the tap at 151 ppm. Do I need to use the reasoning that I need 250 ppm of MY nutes not including what comes out the tap; therefore, needing a total of 400ppm?

I am probably over thinking this, so i apologize! Any help is appreciated!

Just add the nutes needed to bring the total PPM to your desired level. So to achieve 400 PPM, you would need an additional 250, like you have stated.

the PPM on the feeding schedule is just a guideline. Take note on how your plant reacts at different ppm levels starting low and working higher.