RE: DLE 20cc at 6V?
What does the ignition manufacturer recommend?
Fully charge your battery and measure it's output when fully charged. If it is within the ignition manufacturers specs, use it. If not - Don't use it.
FWIW,
Everyone I know that has had problem free running on a DLE (or any other RCExcel) ignition has run a 4 cell NiMh / NiCd style packs OR has had a very good voltage regulator that regulates the voltage below 6.000000000 volts.
Everyone I know who has had repeated problems with RCExcel ignitions has powered them with more than 6.00000000 volts - including with fully charged 5 cell NiMh / NiCd style packs.