I'd rather see the tax around 300 to 400 rDPS if ranged gets additionnal support.
Refresh would be useless in the current healers never running out of MP.
A dismantle like cooldown WITH Tactician would be so broken I'm not even sure 400 rDPS tax would be enough.

At max, 500 rDPS with additionnal support.
At min, 300 rDPS.

You have to take in account that currently the only ranged advantage is Tactician and the mobility. Minne's Nature and healing valse are quite weak.
The melee actually gains dps by having 2 ranged and would probably loose DPS with 4 melee but not enough for the tax to be punishing. I think without uptime strat and doing mechanics as if they were rangeds, melee would suffer a 300 to 500 rDPS at most. But don't take my word, I haven't made the calculations or tried this scenario.