[Rendar]

These are ALL done with rank 5 skills, 2h (with a heavy weapon), and Dual Power.

Shinken

Auto-Enchant! Crimson Clad Elf is enchanted!
--- ROUND 0 ---
Crimson Clad Elf's turn.
Prinny takes 199 Slash physical damage. (Shinken / Rustic Arctic Magical Star)
Crimson Clad Elf's turn.
Prinny takes 199 Slash physical damage. (Shinken / Rustic Arctic Magical Star)
The battle has ended.

Sword skill. Swipes your sword against the ground, creating a gust of sharp wind that will travel up to 5 tiles, starting behind the tile directly in front of you. If it hits an enemy, they take Slash physical damage. In addition, if there is an enemy directly in front of you in the direction you use this skill in, you will perform a basic attack on them. If no enemy is attacked, the projectile's base Range is doubled. (Bonus damage is given to both based on rank.) (120% SWA)

Retreating Swipe

--- ROUND 1 ---
Crimson Clad Elf recovered 10 FP.
Crimson Clad Elf's turn.
Cursed Prinny takes 204 Slash physical damage. (Retreating Swipe / Rustic Arctic Magical Star)
The battle has ended.

Sword skill. A close range swipe as you fall back, dealing physical damage of your weapon's damage type equal to its Power + your STR + a bonus based on Rank, to all enemies within 1 Range, except those behind you. Afterwards, if the tile directly behind you is unoccupied, you will jump backwards to it. (120% SWA)

It's just a change of 5 damage, but even then it doesn't add up.

158*1.2 =/= 199 or 204.

It's 189.6, roughly.

I think these skills may be doing very slightly different scaling than they announce, something more-akin to 125% and 130%. I can't confirm anything, but hopefully you can look into it!

[Neus]

Shinken deals bonus damage equal to rank*2. That matches up with the calculation if it's at max rank.

Retreating Swipe was getting 3 bonus damage per rank for some reason, that's been corrected.