That's not really important. Even if you enter with low levels, you'll want to wait until lv90 before you start boosting your HP. This is because the HP boost bonuses are percentages of your natural HP growth from one level to the next. 200% is twice the HP growth, so naturally, you'll benefit more from using this boost at high level rather than low levels, because you gain more HP per level at 90+ than you do at 50.

Stat bonuses like strength, however, are absolute. A +2 strength gained at level 50 is just as beneficial as one gained at level 90. That's why it's always smarter to do the stat boosts first, then the HP boosts. And what I said still applies, because at the end of Dragon's Den, you will most likely be lv80+ or so. At this point, your strength and magic stats will be so high that your damage maxes out, if you built your characters in a smart way. Therefore, you'll just have HP left worth boosting, and Bahamut is perfectly capable of making your HP reach 9999 before you hit lv 99. You could perhaps get there 5 levels earlier with Diablos, and thus get 10 more speed if you want, but that won't make too much of a difference anyway, as it doesn't affect your evasion or accuracy, only a small increase in atb speed.