This is a good question but not likely to be answered comprehensively within this short forum.
Si is present predominantly as a deoxidizer. But it also provides some solid solution strengthening and hardenability. Without looking at a parcel of specs I would make the general assessment that you will see Si go down for lower temp specs (since it is generally detrimental to impact toughness), down for cleaner steel specs like pressure vessel types (with some exceptions), and up for higher temp specs. It will generally be limited on the top side with considerations of narrowing the austenite range and/or excessive fluidity or even hot cracking.
The specific ranges, IMO, would be determined by a combination of available data, the specific application being considered in the specification, and however the debate amongst the specification committee members with their usually tremendous experience was resolved.
To fully answer this you would need to speak with a long time specification committee member and get a metallurgical history of the discussions involved.