I have given the matter of spelling accents some thought and here is
what I have so far.

We can achieve good coverage of all the necessary speech distinctions
if we use the following accents as a base for a standard accent:

* General American
* Received Pronunciation
* Scottish Standard English

The features of each accent are:

GENERAL AMERICAN (GA):
* Rhotic
* Cot and caught are merged for many speakers
* Father and bother have the same vowel for most speakers
* Wine and whine are merged for most speakers (distinct for some)
* Fern, fir and fur are merged
* Hurry and furry are merged for most speakers
* Horse and hoarse are merged for most speakers
* Mary and merry are merged
* Marry and merry are merged for some speakers
* Sirius and serious are merged (for most speakers?)
* Bath is pronounced with the same vowel as trap
* Strut, foot and goose have different vowels
* Roses and Rosa's are the same; rabbit rhymes with abbot

RECEIVED PRONUNCIATION (RP):
* Nonrhotic (r is not pronounced before consonants)
* Cot and caught are distinct
* Wine and whine are merged
* Father and bother have different vowels
* Fern, fir and fur are merged
* Hurry and furry are distinct
* Horse and hoarse are merged
* Mary and merry are distinct
* Marry and merry are distinct
* Sirius and serious are distinct
* Bath is pronounced with the same vowel as palm
* Strut, foot and goose have different vowels
* Roses and Rosa's are different; rabbit doesn't rhyme with abbot

SCOTTISH STANDARD ENGLISH (SSE):
* Rhotic
* Cot and caught are merged
* Wine and whine are distinct
* Father and bother have different vowels
* Fern, fir and fur are distinct
* Hurry and furry are merged (because they never split)
* Horse and hoarse are distinct
* Mary and merry are distinct
* Marry and merry are distinct
* Sirius and serious are distinct
* Bath is pronounced with which vowel? (don't know)
* Foot and goose have the same vowel
* Roses and Rosa's are different; rabbit doesn't rhyme with abbot

TRADITIONAL SPELLING (TS):
* Rhotic
* Cot and caught are distinct
* Wine and whine are distinct
* Father and bother have different vowels
* Fern, fir and fur are distinct
* Hurry and furry are merged
* Horse and hoarse are distinct
* Mary and merry are distinct
* Marry and merry are distinct
* Sirius and serious are distinct
* Bath is spelt with the same vowel as trap
* Foot and goose have the same vowel (sometimes)
* Roses and Rosa's are different; rabbit doesn't rhyme with abbot

The features that SSE has in common with TS that RP and GA generally
do not have:
* Wine and whine are distinct
* Fern, fir and fur are distinct
* Horse and hoarse are distinct

It follows that SSE has some phonological features that are lost or
mostly lost from RP and GA. Including it in a standard spelling
dialect would result in fewer and smaller changes to TS than if either
or both of RP and GA alone were used to derive a standard spelling
accent. SSE allows us to cater to Caribbean English, which also
maintains the horse-hoarse distinction.