In response to Robin's post on this subject in the Facebook group (I don't 'do' FB):
1. The unit cannot be Open Order, as the rule clearly states that "the whole unit will count as Close Order if it contains any Close Order troops."
2. I agree that some of the wording in the Combined Formations rule is ambiguous, but the second paragraph clearly spells out the basing options and the percentages to which they correlate, i.e. that 3:1 spearmen to archers give you 25% archers; 1:1 (single row of bases) or 2/2 gives you 50% archers; 1:3 gives you 75% archers. On top of that, Martin has mentioned more than once that the Combined Formation rule does not allow you to 'mix up' the spearman and archer figures on each base, and that the options listed in in the second paragraph are the only ones available.
3. However you choose to base your figures, the unit doesn't shoot at 100% just because every base has an archer figure on it. In the example you have given, there are 30 figures on 10 bases. 15 of those figures (50%) are archers (1:1), and so the unit has 50% shooting dice. If it helps, you can think of it this way: 100% of the Combined Formation bases are shooting, but at only 50% strength. 10 bases at 100% strength (no depth bonus) = 10 dice, whereas 10 bases at 50% strength = 5 dice. Perhaps an erratum to this effect is in order, for the sake of clarity.
4. Another possible approach would be to determine the proportions of spearmen to archers for such combined units in historical Crusader armies, and then build your tabletop units to reflect those.