Assuming a smashing/lethal attack hits you for 200 unresisted, according to those numbers you would currently resist 99.4 damage.
So now that's 100.6 damage incoming. With Tough running you would resist a further 35.2 damage, which effectively shaves 35% from the 100.6 damage you would otherwise be taking.
It's really up to personal preference whether you think the slotting of two pool powers is worth that percentage difference... I'm currently debating whether or not to take Tough on my katana/regen. This would mean dropping one of (hasten/resilience/final epic pool power) from my planned build to make room for that damned filler attack that I'll never use...
However conventional wisdom would say that tough woud be a better addition to your build than a SR/Regen scrapper's build... adding tough would currently cut 35% damage from incoming attacks for you, whereas it would only shave off the base 17.6% for someone without any other res buffs.
If you're worried about endurance issues, one thing to think about is epic pools.
At level 40 you'll be able to take Conserve Power soon! ^^
Would it be alright if you posted up a short build, moon_angel? I'll have a looksy if you want 
Just which powers you've taken and how theyre slotted
My martial arts/invulnerability scrapper is at level 40 and at present I am a little dissatisfied with her defensive capabilities. I am pondering respeccing and incorporating Tough into my build.
According to Hero Builder, with three res and three end red SOs, resistance to smashing and lethal would rise from 49.7 per cent to 67.3 per cent, pretty close to the cap for scrappers.
Implications would be having to drop one of the early MA attacks for boxing, pushing a few lower priority powers one slot later... and endurance.
I am particularly concerned about the extra endurance drain, especially as I am already running three toggles.
Any comments?