IMFD doesn't calculate delta-v over time either - it assumes instantaneous thrust. IMFD works around this limitation by re-integrating the course during the burn - so it's constantly making minor corrections on the direction to get the destination encounter time to be the same.
This limitation in IMFD actually makes it harder to use very low thrust engines - IMFD just gives up mid way through the burn because it thinks the current solution can't be met until next go around.
The end result is that IMFD ends up "wasting" a little bit of Delta-V during ejection burns - but with typical engine thrust/mass ratios in Orbiter the effect is minimal.
This limitation in IMFD actually makes it harder to use very low thrust engines - IMFD just gives up mid way through the burn because it thinks the current solution can't be met until next go around.
The end result is that IMFD ends up "wasting" a little bit of Delta-V during ejection burns - but with typical engine thrust/mass ratios in Orbiter the effect is minimal.