I will try to cover some of the factors.
I will use the word BUOYANCY/BUOYANT to describe whether an object/person floats (positive), sinks (negative) or is suspends (neutral) in water.
1) Body composition - this can be experimented with before the use of additional accessories such as wetsuits and weights.
Unfortunately the only thing we can do to change our body composition quickly is to alter the amount of gas in our airways (without the use of accessories or removing a limb!).
If I jump into a swimming pool with a full breath hold of air I will float very nicely.
If I breath out as much as I can I will sink.
My total lung capacity (volume) is about 6 litres, of which 4 litres I can easily get rid of. Those 4 litres make an incredible difference my whether I am positively or negatively buoyant.
I am not suggesting to anyone to use breathing out as a way to control buoyancy underwater - as when underwater you can't get that air back (unless you surface or have an air supply)!
As you go deeper into water the pressure water exerts around you, will reduce the volume of your airways. As such with depth your buoyancy will decrease. The same would go for any accessory on you that is compressible (a thick wetsuit for example). So although you were able to float on the water surface, at 20meters deep you are finding yourself sink without the use of weights.
2) Salinity of the water
Bear in mind that the more salt in the water, the more buoyant you will be.
If I jumped into the Atlantic ocean after breathing out as much as I can - I would sink a lot slower compared to jumping into a swimming pool that has not salt.
Therefore if you plan to test equipment in a pool - you may find minor differences compared to the sea.
The user gave body stats - although this could be used to estimate body composition, and there are calculators out there to work out requirement of weights - the most accurate way would be by trialling it out! Which I know defeats the question asked.
However if you have (or have had) the chance to experiment what you own buoyancy is in water - with and without weights, this gives you a great reference as to what additional weight would be needed for a new wetsuit.
If anyone has a table for roughly what additional weight is needed to achieve neutral buoyancy based on wetsuit thickness and water depth - please post it here. This is just the weight for needed the wetsuit (not the total weight for wetsuit and person).
This may not be possible due to multiple variables of wetsuit materials, size etc.
As I am sure most have found out that the first 5 meters of water require the most weight to achieve neutral buoyancy as airway volume and wetsuit volume are minimally compressed.
The fact that the poster is a tall lean male suggests to me that although his body composition will be low in terms of fat, he will have a large total lung capacity. With a large lung capacity although he will be able to quite easily adjust his body buoyancy between positive and negative, I assume he will want to freedive with a full breath hold and as such need weight even without a wetsuit.
Adding his previous wetsuit on to this will require additional weight.
Changing to his new wetsuit will require further still.
Another way of looking at it would be is to remember how much ADDITIONAL LEADWeight was needed for a previously used wetsuit to achieve the desired buoyancy in the conditions you swim.
If you weigh both your previous wetsuit and new wetsuit separately whilst dry. Then its just maths:
(weight of new wetsuit ÷ weight of old wetsuit) X LeadWeight used for previous wetsuit = estimated weight needed for new wetsuit (based on similar wetsuit material)
I imagine this answer either adds no further to what people know or has made added more confusion to a boring topic.
Put shortly there are lots of variables - in an ideal world someone will have a table with amount of weight needed for wetsuit thicknesses but I have a feeling it might not be that simple.
