I don´t know a formula.
Question is , what do you want for a shield ?
There are fast lightweight shields. = Good for fast movement , with low weight stuff , like armor and weapons , but not very durable , especially against shieldbreaker weapons.
There are heavy shields with high shield body armor , good against all normal weapons.
There are high hitpoint shields, good against shield breaker weapons.
And there are medium allrounder shields. Not too much weight, but very good against normal weapons and good against shield breakers , if they have around 400+ hitpoints.
Short :
- High body armor = good against all weapons that don´t have "bonus against shield" ( like axes , or throwing javelins for example ).
- High Hitpoints = good against all weapons , even shield breaker weapons.
- High weight = bad , for fast movement.
So there are some main trends for shields :
- Some like Steel Shield cause of it´s high body armor.
- Many like the Huscarl shield , cause of it´s good protection radius, and good allround abilities
- Some like faster allrounder like the green heater shield , with only 5 weight , but only 10 body armor and 360 hitpoints , or orange heater shield with 5,5 weight 12 body armor , 410 hitpoints ( same hitpoints like huscarl ).
- And some other shields , that are different. ( Just try )
So it´s about what you like ... try and check the differences.