Ouf. My LexP has about 5 forma in it, and I know some who did a 6 build. With zero or one, it's a lot tougher since your baseline (Hornet, Diffusion, Lethal) will chew up most of your capacity right out the gate. And since you'll also want both crit mods...
Frankly, instead of trying to go half way with 0 or 1 forma, I'd just run a bunch of Lith fissures to get the needed forma.
I just modded it with a high level serration and max rank multishot, it now has about 330 damage rating, and takes down most enemies in 1-2 hits. if I had to forma once, I'd max out the serration, get some good recoil reduction, and/or reload speed, depending on what I feel I need.
Hornet Strike [serration is primary dmg ] Barrel Diffusion , that will be a 30 build. If you give it a catalyst , Lethal Torrent , Bore , and that other puncture mod 90% . Thats the max dmg for a 60 build . If you add 1 forma , throw out the 90% mod and go for crit dgm mods like sharpened bullets and pistol gambit , then use remaining on a smaller version of the 90% mod you ditched . All mods are maxd , in the order shown . I dont have a clue what you have , this is a best build . My Lex looks like this Hornet Strike , Barrel Diffusion ,Lethal Torrent, Bore , Prime Target Cracker , Prime Pistol Gambit , Sharpened Bullets , Hydraulic Crosshairs . It does the business , at 6 forma . There is no room for recoil or reload mods , if you are accurate , 1 headshot will drop 75% of the starmap with a 1 forma build .