Your first port of call would be TRADA, they promulgate the span tables and the latest standard is EC5 (EuroCode)
They also have a decking manual where you can find all of this information.
My thoughts on a comprehensive decking software are… As there are far too many decking types with too many variations in species, material, dimensions, fixing techniques… this would take an enormous amount of time and cost a fortune and just as soon as you launched it it would be almost out of date.
Consider one point alone and that being the posts/joists – would you opt for aluminium, steel, timber (hardwood or softwood) or HDPE (plastic), these all have many different fixing techniques and many different calculations…
Many of the American standards are not applicable in the UK, for instance the standard ledger board is usually fixed directly to the timber of the property with a standard flashing detail – this doesn’t apply in the UK as most buildings are brick or rendered block.
In the case of how wide, long, fixings etc one should really use a timber engineer or commission TRADA to provide the spec, this would then indemnify you should anything fail over time.
cheers for now
Karl