Considering the many formulas available for predicting your 1RM (one rep max), the best way to know which is closer for you is to test it at least once. Many intermediate strength programs have a day to test your 1RM built in as part of the program, and you'll be testing it once or twice per program cycle (roughly 12 weeks on average).
When you get to the point you want to know for sure what your current 1RM is, you'll need to test it safely. Even if you have three spotters, a dropped barbell can happen so fast that they may not be able to catch it in time. So, proper equipment is necessary:
- Power rack: this is a cage with safety rails.
- Safety rails: the safety rails should be put low enough so that it won't interfere with the barbell while you are lifting, but high enough not to cause injury if you drop the bar.
- Flat bench: If you are testing a bench press, you need a flat bench that can fit in the cage.
Now, the rest is about testing the actual 1RM. You will want to have multiple 1 rep sets slowly ramping up in weight. My suggestion, and feel free to customize for the way you want to work, is:
- Start with the highest weight you've lifted so far. If it's your 5RM, that's the starting weight. (You want one good lift)
- Use the lowest calculated 1RM from the equations above. (Brzycki formula: 337.5 in your case)
- Use the highest calculated 1RM from the equations above. (Epley formula: 350 in your case)
- Using your past experience of how you feel, add sets increasing what you think you can above that.
Essentially, you'll have a minimum of 3 sets. You may decide after your second set that you might want another set or so in between. For example, if you feel like 337.5 is pushing yourself to your limits, you might try a small increase of 2.5lb and see if you can do 340. The bottom line is you keep going until you just can't anymore.