Method :

Demast the boat, so you have a bare platform.

Lay the platform up on two sawhorses or chairs so that the rearbeam is aligned perfectly horizontal.

Lift one bow and put that on a sawhorse or chair as well. Then use a rod or plank to bring back the height of the downward flexing bow to the sawhorse or chair of the supported bow, again making sure this rod is perfectly horizontal. Measure the distance between this mark and the bottom of the lifted bow.

I have some measuremnent numbers of some other platforms (boats) stiffness measured in this way; from the top of my head. (1 inch = 25.2 mm)

Tiger F18 / nacra F18 = in the range 94 - 98 mm (platform weight = 130 kg)
Blade F18 = 25 mm (platform weight = 130 kg)
Aging Tornado = 55 mm (platform weight = 115 kg)
Taipan 4.9 (AHPC glass) = 64 mm (platform weight = 65 kg)
Taipan 4.9 (homebuild ply) = 62 mm (platform weight = 65 kg)
Blade F16 prototype (ply) = 45 mm ((platform weight = 65 kg)
Blade F16 VWM 2007 version 90x2 beams = 15 mm (platform weight = 60 kg)

I'm told modern A-cats flex in the range of 15-20 mm when layed up like this. (platform weight = roughly 45 kg)

I have no data on the other VWM Blade F16's but I suspect they are comparable to the ply prototype Blade F16. And of course the weight of the platform must be taken into account as well, although only partially as lighter platforms also witness smaller loadings while sailing and as a result will flex less even when the platform stiffness is identical to a heavier platform.

Also note that while sailing you won't see the above measure of flexing. The test setup is a rather heavily loaded one. In real life sailing it is rare to see halve of it even in very rough conditions.

My own Taipan F16 (modified and widened Taipan 4.9 with a different rearbeam section) flexes rougly 65 mm. As an engineer I would like to have more stiffness but as a sailor I must say that I'm not finding the current flexing as an issue of any sort. I hardly ever notice any effects on the water. Sometimes my beam bolts slacken and then, yes, I feel a degradation in behaviour while sailing in wavy conditions. I don't when the bolts are tight.

If your F16 boat flexes about 2 inches or less in this test setup then you should be content with that. Your platform will have sufficient stiffness for proper handling and sailing. Any additional stiffness will be sexy.

I hope this helps

Wouter