Meth quality is a big factor here. If you took extremely pure meth at a high dose that's not a good thing. Poor, cheap quality meth I can easily go through a gram in a day. High quality meth is like 200-500mg day.
Checking your pulse, can be important, but can also make you paranoid. If it goes above 180 I would be concerned. Also keep an eye on your blood pressure, if you start getting really dizzy or faint that is also a concern. And your #1 concern is if you start getting chest pains or breathing rapidly these are big signs of OD.
People have made some great suggestions on other drugs you can take to chill you out.
One thing that is very effective are certain antipsychotics like risperidone or olanzapine (among many others, almost all of them will replace the meth out of your receptors and will kill the high). I onced OD'd on meth and took LSD as well. I had all the signs of OD and was freaking out hard was about to call 911 then I remembered I had some antipsychotics. I took 20mg olanzapine (zyprexa) and 4mg of risperidone (risperdal) and was asleep within 30 minutes. Completely killed the high.
If you have anyone or know of anyone who has any, might be worth asking them
also, definitely take any beta blockers if you can. Like blood pressure medicine. Propranlol for example.
Overall, I think you should be fine. It's actually quite hard to have a fatal OD on meth. Typically it's people who already have thinks like heart conditions or very high blood pressure and things like that. Worst case scenario you do OD, it's pretty easy to save you at the hospital and generally it's easy to call an ambulance and get help. Unlike heroin/fent OD were you go unconscious and if you don't get narcan within a very short perioid of time they can't bring you back. You can't really save yourself from an opioid OD... you need a guardian angel who can make the call for you.
