Do you have a 5L flask already? If not, then you might want to make two starters. Sure, it will take a few days extra, but you'll actually get more yeast out of it than one large starter.
Starters are best, IMO, made on stir plates. You can get it to finish in about 24 hours. All of my starters (up to 3L so far) have been complete within 24 hours of turning on the stir plate. I would make one, cold crash it about 24 hours, decant the spent wort and pour fresh starter wort on top of the yeast cake in the starter. Get it stirring again and cold crash it when finished (18-24 hours is a good beginning time frame) so that you can decant the spent wort and just pitch the yeast slurry.
I would never pitch an entire starter into a batch. Far better, IMO/IME, to plan a little bit ahead and pitch just the yeast slurry (you'll leave a small amount of the spent starter wort in the flask in order to get the yeast fluid enough to pour)...
Looking at the Mr. Malty site, if you were going with either a simple starter, or one with intermittent shaking, then going with a stirplate method will mean a MUCH smaller starter. Depending on how fresh your yeast is, probably in the area of a 2L starter (depends also on the actual amount of wort going into primary). Use the fermenting volume for the calculations, not your finished/into bottles volume.
BTW, are you kegging or still bottling? If you keg it, you can assume 2 weeks to carbonate, not 3+ weeks from bottle carbonating. Which means, if you brew it next weekend, you can leave it in primary for a month, keg, chill, carbonate, and have it ready (on tap) by 5/5. You could even go 5 weeks in primary, keg it and it should be fully carbonated (and chilled) by 5/5. Just something to think about.
