Two successive paroemiacs were admissible only in anapaests of this 'free' or lyric character (n. on 86—120). These paroemiacs are purely spondaic; as 'free' anapaests also admitted the converse license, of resolving the long syllables, except the last, of the paroemiac.
Source: wiktionary
The metrical differences between march and melic anapaests are differences in respect of feet, caesura, continuity, use of the paroemiac, and catalexis.
Source: wiktionary
It might be objected that the ratio depends on the exigencies of the drama, that e.g. short anapaestic passages of only half-a-dozen or so metra plus their paroemiac might be required in some plays, not in others, and thus lower the ratio for those plays.
Source: wiktionary