Salt Tabs???

What I did is first did a sweat test. From there you can estimate what you lose in sodium based on the normal 400-600 mg/per 16 oz of sweat.

See what you are consuming from other sources then back into what you need to supplement.

I used Lava Salts as they are 420 mg each and I have a high sodium need.

I work with a coach and we started me low and I kept cramping and gradually upped my goal sodium amount until it works, I no longer cramp and it seems to balance. I take slightly more when it is hotter as I sweat more and lose more sodium. 

I use salt tabs (Hammer Endurolytes) on long training days and races.  More in hot weather, less in cool weather.  I just did IMCDA and took 6 on the bike (bike split was about 5:40).  It was pretty cool on the bike, never above 74.  Last year in a HIM where the temps reached into mid-90s I took 6 in less than half that time on the bike, so it is weather dependent. 

You should also be looking to get some salt from your liquids, whether that's gatorade or adding NUUN or another electrolyte supplement to your liquid nutrition. 

One last thought, do it in training (a lot) before the race.  Some people have stomach problems with sodium.