If you only cook them "kinda rare" and then hold on a sterno rack, they will be tough, unless you can shave them into very thin slices.
I would aim for "quite rare" and count on the hold to further cook them.
Six 5 pound roasts!! That's 30 pounds of meat!....will your oven hold them? Unless you have good air circulation around the roasts, you will be best figuring on the time for a much larger piece of meat.
I have never done chunks of meat that latge all at once in 1 oven. But based on what I have done, I would cook at 425 for an hour and then test with the meat thermometer, and aim for 125 to 130....maybe even lower temperature.
I have cooked 20 pounds of bottom round, but I had the butcher tie it all together so I could cook it as one very large piece of meat.

Good luck!!
Linda C

Linda I have a GE Profile Performance w/convection....do you think Id be better off cooking 2 at a time to RARE in the center rack...Id hate to screw this up as this is one of the dishes for DH 40th bash....hate to admit but theres a lot of potential new customers for my business at this party so Id hate for the meat to be overdone. My SIL Rosanna had the same opinon as you RARE...because it will finish its cook on the rack ...Another question as far as the other 2 back up trays (there are 3 trays each will have 2 thinly sliced roasts in each) the staff will be warming all the foods for the party an will set the buffet with one of each tray of foods then there are 2 back ups per rack. How shoul they heat the other 2 in the oven so they will be hot enough for a switch when the 1st pan is done...so not to be overcooked? I sound crazy I know but Im more nervous about this party then any other one I have ever done! Thanks

I would not warm a whole pan at a time for the backups....you don't know how fast people are going to empty out the trays....and the meat won't improve in taste nor tenderness by sitting in the oven.
I think I would be holding the extra roasts, unsliced in the oven and having the staff slice them as the first were being served.
Any way you can beg or steal...or borrow an unfra red warming lamp?
Linda C

Eileen, I hope you will let us know how your DH's party turned out.

I will let you know its a week from today an im cooking and entertaining 80 guests I hired 3 staffers to work the kitchen that evening. the theme to the party is LAs Vegas Rat Pack era the hall is being decorated with vintage pics an posters of the Rat Pack the DJ is only playing classic sinatra jerry vale jimmy roselli etc...all who are attending are dressing semi formal...I cant wait very excited!

Sounds like a bash!!! Lots-a booze? LOL! That was a hard partying crew!
I went to a "show" a couple of months ago, one of those travelling shows that fill in at big houses for shows that for one reason or another were cancelled. It was called "The Rat Pack in Vegas"...or something very similar....and those singers were dead on!! You could have closed your eyes and thought youw ere listening to The Chairman in person!
So....what ate YOU wearing??
Linda C

Sure theres open bar and I made my own Limoncello which has been marinating a month in the freezer,bought little mini martini glasses to be tray passed during cocktail hour. I bought a beautiful black chiffon Maggy London dress that has a definate retro 60's cocktail classy look. Bought a new pair of dynamite Nina black sling back from Nordstroms I will try to post some pictures afterwards.