I think the rule is a poor way to judge a performing corps. If number alone must be used the number is too high.
That being said, it is what it is for now, and all you can do is hope for more in the future. But...
Regardless of the rule - don't let it be anything that would stop you from going to Finals. Plan on it. Put it on your schedule today. Don't say "We might." Say "We are." And if the chips stack up that you don't have enough people to perform a field show, then put together a Minicorps, 15 solos, 3 ensembles, and a Chorus and have the time of your life at I&E, then have a ball watching two days of competition on the field, hanging in the lot listening to warm ups, and lurking in Souvie Row. Watch it all, check out the Alumni show, be a part of the weekend. Doing so will ensure that the folks who you've spent so much effort recruiting will be envigorated and want to go again next year. Starting out by saying "We might go" sounds far too defeatist to me, Charge forward knowing that it's where you want to go, whatever performing opportunity presents itself. Push your members into performing and let the bug catch them as it has you. The trip could make the difference in the future of the corps - on the field or not - it will be a moment that cements the group together and starts building the kind of program that you want to have.