Sigh.
Assuming attack is somewhere around 1000, and that you're using Anchorites for 20% Boost.
When getting haste:
If cRatio is roughly 1.85 (ie: mob defense no lower than ~540), then a boosted Smite will end up with nearly identical DPS as an unboosted Smite. That cRatio is the point where adding Boost takes you right up to the 2.25 cap.
As the unmodified cRatio increases above that point, the excess additional attack is wasted, reducing its overall value to below parity.
As cRatio drops from that point (ie: fighting tougher mobs), Boost gains value relative to DPS parity.
When you add in double Marches:
Against mobs with no level correction, it will never be worthwhile to use Boost.
Against mobs with significant level correction (eg: Ig-Alima), it is worthwhile to use Boost until the attack cap is reached (ie: when def down reduces their defense below about 510, given the same setup as above).
Without any outside haste, you have additional clearance going beyond the 2.25 cap. That is, you can 'waste' some of Boost's effect and still have it be useful. For the above scenario, you're good down to a mob defense of about 480 (what would be a 2.5 cRatio if you didn't hit the cap) with level correction, or 510 defense without level correction (~2.35 uncapped cRatio).
Since mobs of interest are generally only Adoulin mobs, or are low enough that level correction no longer applies, we can assume there to be no level correction for the general rule. As such, the summary is:
With Marches+Haste: Never Boost
With just Haste: Use Boost when below 1.85 cRatio (ie: mob defense of 550 or higher)
Without Haste: Use Boost when below 1.95 cRatio (ie: mob defense of 520 or higher)
Scale the defense values to account for your actual attack.
Note that the level 102 Chapuli (found in Sih Gates, immediately connected to Ceizak) has a defense of 534. You can expect mobs in higher level Adoulin areas to have notably higher defense than that.