Simply measure from the centre of one spring bar or tube, across the watch (north to south) to the centre of the other spring bar or tube. This is your Lug-to-Lug measurement.
On receipt of your strap you may initially think the keepers are too close together, they are not. As a strap is wrapped around the wrist the keeper tabs ‘pop’ out to roughly what they need to be. I need to do this to stop the strap bunching up under the watch head. Its not an exact science and there is some waggle room in the nylon when fitting.