depends how clean it is, if its the browny-orange dutch rocky/shardy stuff you'd be safe with up to 200mg swallowed on your first bomb imo, i wouldnt go for that much of any chemical in the stomach without a good amount of food/drink though, it could give cramps. if its the bright yellowy-white lemony chinese stuff, slightly oily (not the pure white dry crystal), go for 100mg-150mg, and if its dry white pure crystal, its probably NOT real mephedrone, and i either wouldnt bomb it (sniff little bumps incase its dodgy, some mephedrone replacements can have a mild percieved effect with a very high potency chemical, so you take more and more to get past a certain point which you physically cant get past, and thus OD)
and before bombing any chemical i allways start with a little bump (nasal) just to pick you up and prepare for the onslaught.
unless youre sneaking into a venue or something, just sniff the shit its much better.