ZFW Controller described a proximity event after transferring two in trail aircraft to D10; the reporter indicating compression separations could be better judged with improved HOST speed indications.

Narrative

Air Carrier X and Air Carrier Y were being sequenced into DAL. I let the aircraft compress to about 8 miles and then slowed Air Carrier Y to 250 KTS behind the Air Carrier X to get the required spacing. I then switched Air Carrier X to Approach Control. I had Air Carrier Y stopped at [12;000 FT] for additional traffic and told him to reduce speed to 250 KTS; then descend and maintain 10;000 FT to keep him laterally separated just in case the speeds were not compatible with the Air Carrier X. When I saw Air Carrier Y leave 12;000 FT; I had 7 miles spacing and told the Air Carrier Y to cross FINGR at 9;000 FT per the Letter of Agreement with D10. I then switched the Air Carrier Y to Approach and was told after I got off position that I had had a PE. I knew that I had done everything right and was rather upset that they were determining this [to be] a 'deal' since I believe that D10 slowed the Air Carrier X to 210 KTS to let the two aircraft compress in their airspace. I was told we were not going to call D10 to see if this was the case because it was only a PE. With HOST not showing immediate and accurate speeds [it] becomes extremely difficult to use spacing when one aircraft is being slowed. If we could get HOST to better show speeds it would be a great benefit.