Isanths are NOT a good analogue. My best suggestion is taking your own designs and pitting them against each other.
When I test, I put variations of my ships up against each other in varying amounts based on size and mass, and note anything that happens with the AI since all my stuff is focused on AI-useability (Hence why all my things currently lack Ion, Overdrive, and large ships using Power Aux havent been created yet).
During first tests, I shoot into inert targets. I use huge target "blocks" initially, then test weapons against some of my actual ships spawned in idle mode. They make great "worst case" targets for being struck with missiles onto their double layer Spaced ADV armour as the blast gets absorbed quite well.
Generally, I want weapons to at least reach damage threshold limits. So at the least a small gun needs to deal 100 damage to 1-shot a weapon block every shot.
Next is with Cannon/Cannon/Punch, I aim for the system to at least get 3 penetrations which happens at 'around' 100/100/100 CCPu. The shell damage in the thing per barrel should say something like 125, 63, 27.5. with 25 being an important number to beat as this is the HP of a capacitor/reactor/thruster block. A cannon like this will destroy a an ADV armour block in a few hits to the same place. However its not as efficient as something a bit bigger. Also note that Every new weapon output added onto a computer multiplies power use by x1.10, meaning a 10 output system consumes 2x the power than if it was 1 barrel. Do not mix with Overdrive.
For missiles, their break-points are more set in stone (More for larger scale systems), and based on the composition of blocks in the target. I almost solely use Missile/Beam/Piercing due to piercing effect on missiles damaging shields, AND provides latent anti-armour and +5 radius to the damage. (And rarely missile/pulse/pierce)
The only time Piercing is not an advantage over Explosive or over Punch effect is when the target is either raw system blocks, or made of basic materials like motherboards or basic hull which offer little protection (Though, hulls offer a missile-resist effect due to AHP)
Generally, a lot of my "testing" is just done by checking weapon numbers and seeing if they are what I expect, etc.