Wait until the adset has at least ten purchases or conversions at or below your target cost. Ten is the number I want before I touch anything. Once it hits ten and the cost is where I need it, I start scaling slowly.
Slowly means five to fifteen percent every three to five days. If you are nervous, stay at five. There is no prize for jumping it forty percent overnight and watching it fall apart.
Raise the budget in the morning, not midday and not at night. facebook can accelerate spend to catch up for the hours it missed at the new budget, so the earlier in the day you raise it, the less risk of a spend spike. I am not staying up until midnight for this. I raise budgets when I wake up and it paces fine.
After you raise it, do not judge the day you raised it. Start looking the next morning. If it paces at the same cost or better, you raise again in three or four days. Then again. Then again, until you find the point where the cost climbs past where you want it. That point is your ceiling, and you will not find it by guessing. You find it by raising.