The tail current percentage will not vary with battery Dell Latitude D520 capacity.
Either accept the default 4% or maybe take it down to 2%, otherwise the generator will never turn off, assuming its triggered to do so at 100% indicated SOC.
A lot depends on the float current of your fully charged bank which as pointed out, clearly needs to be less than the current calculated from tail current percentage of bank capacity.battery ASUS N71JQ
If you are concerned that the generator won't be commanded on until below 50%, raise this threshold to 55% or 60%. TBH if I had an automated system such as this I would have the generator powering up at a much higher SOC, say 70% to improve battery life. I appreciate a balance has to be found though to prevent excessive cycling on/off.