Looking at the map, we reckon that the station must have been roughly at the end of Duncan's Lane rather than at Holden Road - and this was near the Caroline Chisholm Shelter Shed which was near the intersection of Duncan's Lane and the road to Mount Alexander.
I have only been looking at current maps, can take it Duncan's Lane is now Duncans Rd? Is the alignment of the current Calder approximately that of the road to Mount Alexander? Presumably Duncans Lane continued on the same alignment across the current Calder alignment, then the railway and intersected with Holden Rd?
One thing I forgot to look at before is the land parcels where you can commonly identify former station sites as a widening of the railway reserve, in this case there is a widening of the reservation from a few hundred metres south of Holden Rd to a few hundred metres north of a presumed intersection of Duncan's Lane. The station was almost certainly within those confines, but because it is so long it doesn't really help.
The distances I gave you (Sydenham 15 miles, Holden 18 miles and Diggers' Rest 20 1/4 miles) are accurate to a quarter of a mile or so, but this does leave some leeway as to where the actual site of the station is.
I will certainly keep an eye out in the future for anything referencing Holden.