https://www.bg-wiki.com/ffxi/Tactical_Points
This can be used to calculate TP, under the first heading. First, you'd calculate how much dual wield you need to cap delay by using:
(1 - Haste amount) * (1 - Dual wield amount) = 0.2
Say you have 68.3% haste, you'd do:
.317 * (1 - X) = 0.2
Divide both sides by .317
(1 - X) = 0.63
Solve for X:
X = 0.37
So, you need 37% dual wield to cap delay. Now, calculate your single weapon delay using 37% dual wield to see the maximum TP per hit you could gain while still delay capped.
Delay formula:
((weapon 1 delay + weapon 2 delay) * (1 - DW) / 2)
TP formulas are in the chart on the link I gave, and change based on delay range.
Finally, redo the last 2 calculations using the actual amount of dual wield you have, and you'll be able to see how much you're losing per hit for no additional gain in delay. It's usually not a large amount.