The modern trend for "micro" frames simply displays eloquently the triumph of the marketing might and muscle of certain large and influential bike manufacturers, over common sense... or that's my opinion, and I'm happy to chuck my hat into the ring ! Not that there no advantages whatsoever in the small frame concept... it's just that there are too many disadvantages.
In the late 80s/early to mid-90s I was very involved with the development of carbon fibre frames and worked along with companies such as TVT, RBE, Guichard, Ribeaucourt, Mach 3, and Corima in France. My involvement with Corima and arranging the supply of the monocoque on which Chris Boardman broke the World's Hour Record at Bordeaux in 1993, led me to having a lot to do with Mike Burrows, the Norwich-based cycling engineer who had developed Boardman's Lotus bike for the 1992 Olympics.
As a result of his involvment with Chris, Mike became a design consultant for Giant of Taiwan, at the time that that company had decided to move up into manufacturing value-added high-end machines. I recall being in Mike's workshop when he started experimenting with the "next" generation of road bikes.. for which he "invented" the "sloping concept". By his own admission, the design had very little to do with efficiency in use of the frame, but more to do with manufacturing efficiency. By convincing everyone that a smaller frame is lighter, stiffer, rides better, etc etc and that " three sizes plus long seat-pins fit all", Giant managed to improve output and turnover, and reduce the amount of stock sizes it had to manufacture and keep. And the rest is recent history... as the saying goes.
I don't think that there ever has been a universally formula for calculating the "correct" size of frame for any individual... and I suppose that different builders have different notions. I can only speak for generally accepted principles for Europe and have no idea whatsoever about what applies in the States. However the generally accepted formulae, in round terms, in Europe are/were:-
England(50s-80s) inside leg measurement minus 9" (particualrly for club and touring riding); perhaps a little bit more off for racing; France (60/80s) " " " " 25cms Italy (50s/80s) " " " " 10" ( or metric equivalent) using the CONI method France ( 80/90s) " " " multiplied by 67 percent (method used by Hinault and the Renault team)
The Var "Little Yellow Book" gives Eddy Merckx's frame as being a 59cms with 57 top tube, Zoetemelk's as 54.5/54.5. and Van Impe's as 50.5/52.5... and not a "sloping concept" amongst them.
Some years ago, in the mid-80s I was given the opportunity of buying Coppi's 1952 Tour de France bike for about £1200.. but couldn't afford it. I recall that it appeared to be about a 24"/60cms sized frame. Bearing in mind that Coppi was only 1.76m tall, that was a very large frame...but he did have long legs! Would he have gone better and been more successful on a "micro sloping"? That .. we shall never know.
a couple of years ago or so I was lucky to get a chance to talk to quite a few riders about to take part in the Liege-Bastogne-Liege classic, about their bikes, and in particular the "sloping" frames. Quite a few of them disliked them intensely saying that they always felt as if they were sliding off the back of the saddle!
Norris Lockley... seldom getting the chance to get on the saddle these days, let alone slide off it... Settle Uk