If your Maths is up to scratch then it’s not too difficult to work out BC from 1 Chrono
Calculating BC from Scratch Using Measured Data
You can calculate BC using a chronograph and shooting at least two targets at known distances.
Step 1: Measure initial velocity (Vi) using a chronograph.
Step 2: Shoot a target at a known distance (D1), and record the bullet’s velocity (V1).
Step 3: Shoot another target at a greater distance (D2 > D1), and record the bullet’s velocity (V2).
Step 4: Calculate average drag function K for both distances:
K1 = (Vi – V1) / (0.5 * rho * A * C * D1)
K2 = (V1 – V2) / (0.5 * rho * A * C * (D2 – D1))
where rho is air density, A is bullet’s cross-sectional area, and C is the drag coefficient of the reference projectile.
Step 5: Compute the ballistic coefficient using the average of K values:
BC = C / ((K1 + K2) / 2)
Much cheaper than a 2nd or replacement Chronograph. Once you have this set up in your device, it will then be very simple to get the result, likely more accurate than 2 chronographs unless they are reliably accurate and consistent between them (good luck with that). You can of course sell your Garmin and buy a replacement at twice the price. Hopefully you’ve checked out the reviews?