Mithral armor worth it for multiclass fighter?

[soulnova]

I'm planning my PF society character and Im wondering about armor.
Is mithral breastplate worth it for a multiclass Fighter that needs to tumble to flank? I will get Armor Training later than usual because I'm dipping into two different classes and I will likely need to move through threatened squares to help flanking so acrobatics are important to me.

[Lelomenia]

What do you expect your dex to be at levels of interest? Basically, mithral is almost always better than anything else, and breastplates are better than most other armors but if your dex is extremely good or extremely bad a different (mithril) might be better.

[soulnova]

She can afford the Mithral Breastplate at level 4 but won't get armor training until lvl 5.

She's starting with DEX 14 and I assume she will get it up to 16-18 with magic in later levels/help other chars.

[avr]

Assuming you have acrobatics as a class skill and you keep it maxed, your skill should be character level +5 before ACP, +6-7 once you increase Dex. A masterwork breastplate has ACP -3, mithral has -1.

Enemy CMD averages about 19 at CR 4 and about 34 at CR 11. A mithral breastplate gives you a 50/50 chance at level 4 vs. CR 4 and about a 25% chance at level 11 vs. CR 11 (assuming armor training 2, Dex 18, but no other items adding to acrobatics). If PFS usually throws you against many lower CR enemies those figures get easier; a masterwork breastplate makes them 10% worse.

A mithral breastplate is useful but not sufficient IMO. You might prefer to spend the money on boots of elvenkind (+5 competence bonus for 2500 gp) first.