Different battery cell types from different manufactures have different discharge profiles. I would cycle the battery and watch the the voltage drop and make note of the Ma's discharged on the charger as the battery  gets closer to the bottom. The Rcexl ignition draws about 500 ma at WOT and will work reliably at voltages of about 4.5 volts or a little less while under load. The older versions of the Rcexl ignitions will work down to 3.5 volts before they start cutting out at high RPM.

Just using an arbitrary voltage number is not the best way to judge whether a battery is safe to fly or not unless you are familiar with the specific battery. I would also strongly caution against following any individuals advice on a voltage number on any forum as there are many different battery types in use as ignition batteries.

Cycle your battery a few times and learn it's discharge characteristics, don't fly if your battery is at a level of discharge that won't hold at least 4.7 volts for about 20 minutes with a 500 ma load.