Here's some math (from the FFXIII game mechanics guide by ximaus) -
"____________________TP Bar
_________________________________________________________________________
| Lv1| Lv2| Lv3| Lv4| Lv5|
|====|======|====================|=====================|==================|
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| 100| 200| 600| 600| 600|
| | | | | |
| | | | | |
|<---------------------------- 2100 ------------------------------------->|
The TP bar is a total of 2100 points long with the first 2 bars being 100 and
200 points respectively and the last 3 bars being 600 points each.
TP abilities use the TP of only the full bars beginning at level 5 and
working its way down to the level 1 bar. In other words, TP abilities cost
more TP if your bar is full. For instance, a summon ability used at TP level
5 will cost 1800 TP or rather, bars 3 through 5. The same ability used at TP
level 3 will cost 1000 TP, or bars 1 through 3.
If you happen to have Bar 5 half full when you use a summon, the summon
ability will completely use bars 2 through 4 and the remaining half will fill
bar 2. In this case, even though Bar 5 had 300 TP, when it is converted to
fill Bar 2, it will only fill it by the same percentage. This means that you
will essentially lose 200 TP since Bar 2 will only fill by 100."
*Party recovers TP after battle
"*____________________TP Recovery
The amount of TP recovered after each battle is based on the enemy level and
the Battle rank at the end of battle.
Rank Recovery
0 stars Sum of enemies' levels / 8
1 star Sum of enemies' levels / 4
2 stars Sum of enemies' levels / 2
3 stars Sum of enemies' levels
4 stars Sum of enemies' levels * 3
5 stars Sum of enemies' levels * 8
Enemy levels are not intuitive in this game as even early bosses have levels
above 50 while one of the strongest enemies in the game is only level 7.
Levels are more of a battle calculation factor than an indicator of ability
so keep in mind that you may very well gain more TP from a group of weak
enemies than from a strong enemy."
"*Other methods of TP Recovery
Method Recovery Amount
------ ---------------
Party member defeats an enemy 2(*1)
Party member with Energy Sash defeats an enemy 12(*2)
Party member fully stacks their ATB gauge and 4(*1)
damages, recovers, or inflicts a status effect
Fang staggers an enemy with Punisher equipped 10
Fang Staggers an enemy with the upgraded versions 20
of the Punisher
Use an Elixir in battle Full
Use an Ethersol in the field Full
*1
During a summon, recovery amount = 0
*2
During a summon, recovery amount = 10
If a party member has the Synthesized Ability: Gestalt/TP Boost and is not
dead, all listed values become double; this doubling affect the actions of
all party members, not just the person with the ability. If the recovery
due to the increase from Gestalt/TP Boost becomes more than 10, then
recovery equals [(2 * normal) - 1]."
Yeah the idea here is that Vanille/Fang will start of a floor with 1.5 off the full 5.0 to begin with and hopefully build up another 1.5 from gaining TP after fights/actions in battles/etc to hopefully make it to the 3.0 needed for a Summon by the end of a floor =P They will carry over the TP they've earned to add to the TP they start with between fights but not between floors. Seems reasonable to me? Not particularly broken or anything since they don't pick up their summons till late (yeah they do get MT Full Healing/Status Recovery/Revival in one when they do but it's use is restricted)
* Clear Tranquil will look up ATB segment gains later