On the subject of larger diameter turntables and roundhouses, the angles between the roads for No.2 Roundhouse at Broadmeadow is 7degrees 49minutes & 34 seconds, the same as for a 75ft turntable. Admittedly Broadmeadow No.2 turntable roads were in place from the original 1926 vintage 75ft manual turntable, which was replaced by a 100ft (the same as Junee) example when No.2 shed was constructed in 1948/49. The 100ft turntable was enlarged to 105ft in 1951 to suit the AD60 Garratts. No.2 shed at Broadmeadow is very similar to the East shed at Junee, the foundation drawing for Broadmeadow No.2 shed actually makes several references to the drawings for Junee.
There is more to it than that, especially when you are working off plans, & the various depots as well. The other aspects is whether or not the larger Turntables were placed in service at the same time as the RH, the other consideration is whether you are looking at the more common Saw Tooth roof alignment or the high pitched venting roofs.
Most Round houses were built to a standard design, dependant on the roof type, likewise the size of service pits. Junee being the most modern of all is unique in several ways, the brick construction, & the High roof venting, same as PTW, Valley Heights, 3 shed Enfield, 2 Shed BMD, Muswellbrook, Temora, Grafton, & Taree.
The Saw tooth types were located at BMD 1 shed, WCK, Enfield 1 & 2 shed, Thirroul, Goulburn, Cowra, Orange, Lithgow, Kempsey, Broken Hill, Parkes, as well as a couple of others.
Junee roundhouse East shed had 81 foot centre to centre post front to rear, which was the common size of the saw tooth types. The Turntable issue is that in the main, most depots started with 60 foot TT's with normal rail spacings, those later modified to take 75' TT's had the pits extended to take them, the same happened with those having 90' TT's which had the pits enlarged.
When modelling comes into play, & having gone over quite a few plans, including one that floats around on the net, that shows a 75' shed, 75' pits, & 75 TT, that is unusual indeed, it was the original plans show a date in 1922, amended 2 odd months later, another amendment in 1924, & another one in 1952, that later amended plan is what is found on the net, yet I do not know where that is located, or if it was actually built owing to the time frame, & dieselisation would render RH's superfluous.
The thing is though when considering building a RH to plans & the angles, it would often mean that the average modeller would not likely have the room to build a large one, the one I am building is only of 8 stalls, & no matter how I set the angles, I have to also consider that the TT I am using may well be different to the standard NSW types.
When looking at the plans for the standard saw tooth RH, in Byways 7 of Broken Hill show the centres on the rear wall show them as just over 32' which is large & BH had a 60't TT, yet the angles of the roads suggest that they appeared wider than normal, while the net 75' has a shade over 23' centres, same as found on the Pitched roof types.
As I worked at Enfield & WCK for years, also went to Lithgow, Goulburn, Thirroul, BMD on a lot of occasions, which I wouldn't mind a few dollars for each visit to them when working, plus going Parkes on loan, & PTW, I do have an idea about them & the methods of construction. Certainly Junee is different on many ways also, with the two sizes of the one Roundhouse, The use of concrete piers also. Certainly it was the most modern of all NSWGR depots likewise the 100' TT was unusual for NSW also.
The wiki screed that shows Thirroul which was posted earlier, also shows the depot as having a 75' TT, yet no mention of it being enlarged to cater for big engines working there from Enfield.
The one common element though in all of the RH's that I can find is the general size of the windows along the rear wall, & that is what was asked in relation to the Thirroul RH.