Critical setting-out of steel posts to support large decking sub-frame - advice needed

[B52s]

Looking for some guidance as I'm not sure how best to set-out steel posts (to be set in concrete) to ensure that the 150x150 top plate of each of the 16 No. box section posts will match perfectly with the L-shaped designed decking sub-frame to be supported and bolted through onto the top plates below. A friend has given me a loan of his Dewalt Crossline Laser DW088 to assist with the setting-out procedure, but I'm not confident that this will be sufficient, or the correct tool to use, to achieve the critical setting-out of the posts required.

Any advice would be greatly appreciated. TIA

Edited 1 hour ago by B52s

[Nickfromwales]

A laser with 2 beams will give you a perfect x & y, so you’ll need to set out from one straight line (with that as your x) and then you can reliably cast the 90° perps (y) off that. 
 

Then it’s just down to measuring everything twice.

 

Do you have a grid plan you can post up? 

[Russell griffiths]

Don’t set them in concrete, bolt them onto a concrete pad, this way you can build all the structure up and bolt them down when you are happy. 
too many things get drawn up with little thought as to how to build it. 
 

or you need two wooden posts and a string line. 
 

or the alternative method is to dig your holes and build the entire structure, but don’t concrete the posts, just hang them in thin air by putting temporary blocks under the structure. 
when you are happy it’s all correct get a load of concrete at once and concrete in all the posts at the same time. 

Edited 28 minutes ago by Russell griffiths