Shimano Rear Disc Brake Mounting Bolts

I need to buy some long mounting bolts for a BR-7170 rear disc brake caliper. From what I've picked up so far I need what is called a Type C bolt and a Type C2. These mounting bolts come in different lengths, on one site 10mm, 15mm, 20mm, 25mm, 30mm and 35mm. The thickness of the mounting point on my frame is 25mm. I'm guessing therefore that I need either the 30mm or the 35mm. As this is to mount the rear brake I'm hoping there is a more factual method of assessing what I need rather than my guess work. Any advice appreciated.