Most of my clams (7) are either 3" or larger with the exception of a 2.5" Crocea.
I've never target fed my clams due to 1) the 2.5" Crocea is attached to a good sized piece of live rock. If I bump into this piece, let alone pull it out for target feeding, I would probably dislodge a whole lot of other corals. 2) I simply do not like to constantly handle the clams. They seem to "settle" into their comfort zone position & here comes the giant hand to pull them out of the water.
I do feed the tank though with alternating DT's (5ml per 15g of tank water) & Knop's Koral Vit F (1ml per 22g of tank water) twice in a 7 day period. I only dose the recommended dosage even though DT's recommends slowly doubling the dosage for clams. I don't know if the clams are getting enough of the live phyto from the DT's or enough yeast based formula from the Knop's but I tend to think that 90% of their intake is from the lighting. Although this might not be true for clams under 3", my 2.5" Crocea has been doing well for 8 months without target feeding every other day.
Even with the twice feeding per 7 days, I do have some nussiance algae build up. I suspect this might be from the DT's, I've heard other reefers complain about this and eventually stop feeding all together. I did not have a nussiance algae problem prior to feeding the tank. I'm sure my SPS & zoos are taking in the nutrition too. I'll test the theory as I'm about to run out of DT's phyto, I'll continue with the Knop's Koral Vit F & see if the nussiance algae goes away.
Of course this is my own opinion and it has worked well for me, others will probably disagree. Actually, in all this time that I have been "feeding the tank", I can't even tell if the clams are taking in any of the food sources, they really don't seem to react differently when being fed.