Referee style, yellow fiber glass folding ruler.
1 meter ( 3 1/4 ft) long, and the last part can be extended for an exact fit.
No need to worry about inches or millimeters: just fit the ruler between one of the boules and the target ball. Then do the same with the other boule(s).
The answer will be obvious.
If you order only small, light, accessories we can usually ship in a padded envelop at $4.95. We will adjust the cost BEFORE charging your card or Paypal.