You can start with your bullet maker's BC, as correlated with any other sources you can find.

This is often 'rough' because they are not generally qualified.

For example, the maker might list a BC of .460 for your bullet, but without mention of velocity or atmospheric standard(much less bullet lot#), this value was based on. It was probably estimated through math rather than field validated which could introduce a small error as well.

So for you it's a generalization about as accurate as you could eyeball yourself..

But it's a start, and Berger and Lapua are advancing this process for us. Someday it won't be so difficult. Berger has lately listed BCs based on average velocities common to a bullet's use, and this is about the best information available off a bullet box.

For long range bullet's, Bryan Litz(with Berger) has measured, calc'd & tested, and provided results in a very good book:

Book
Muzzle velocity is needed for sure to convert G1 BCs to G7, so plan on doing so AFTER load development across a chronograph.

To correct your BC, you could use 2 chronographs to get near & far velocities for calculating BC. This has also been done with an Oehler setup that measures 'time of flight' with a triggering chrono + downrange microphones.

Or you can shoot several distances and tweak your BC inputs to ballistic software for the closest match at all three distances. But counterintuitively, this is actually the least accurate method.

Exbal has a BC validation screen for this, and LoadBase & Pejsa's provide a process for this method as well as the others.

For a 175smk:

Sierra list's it .505@2800fps+

Sierra Bullets - The Bulletsmiths
I believe Sierra bases this on Standard Metro atmosphere, and converted to ICAO atmosphere this BC becomes .496

Your version of Exbal is one atmosphere standard or the other(or another) so be sure to understand this during use.

An ICAO G1 BC of .496 @ 2800fps converts to a G7 BC of .248 with same qualifiers.

You can do this here:

JBM - Calculations - Drag Function Conversion
Bryan concluded a little lower G1/G7 BCs for this bullet in his book. He also provided averaged BCs throughout 1500-3000fps as .475/.243

(I hope to morally provide this info, and not burn in hell)