This is the best guide I've found. It's crazy that there's no formal info out there describing the new system and the numbers behind it.

If focused on their specialty a Tier 3 national scout will give you 80%, 70% if they're scouting a region. T2 national will give you 75%, 65% regional. T3 national and a T1 regional same specialty will give you 99%. You also get 6 private workouts / focus players for another 40%.

In the post above he stacked a T3 national WR scout and a T2 regional WR scout to get 13 WRs to 100% in the T2 region. The workouts and focus players got him another 6 WRs to 100%. 19 of 51 WRs 100% scouted but all other positions only scouted 40-59%.

If you are scouting two positions T2 national and T3 regional will give you 75% for the national position and 70% for the regional position. Use the workouts / focuses to pick 6 of those players to get to 100%.

All of that said I don't know what's best. You can look at a 70% scouted player and know he's great/elite without going to 100%. Story lines and tags seem to indicate dev traits more than anything.