Sales/Marketing’s budget (S of SG&A) is referred to as the marketing communications budget (MCB) in the iconic marketing text, Marketing Management, Kotler et al, 15th edition, 2020, pages 551-557.  In that text, three best practice steps are described.

1. Establish the total market communications budget: “We will describe four common methods:
   1. Affordable:…set the communications budget at what they think they can afford…completely ignores the role of marketing communications…
   2. % of Sales:…set communications expenditures at a specified percentage…of anticipated sales…has little to justify it…
   3. Competitive parity:…set communications budget to achieve share-of-voice parity with competitors…problematic…no grounds for believing competitors know better.
   4. Objective-and-Task:…has the advantage of requiring management to spell out assumptions about relationship among dollars spent, exposure levels, trial rates and regular usage.” For details, see Marketing Communications Budgeting, page 552
2. Select the marketing communications mix: “Companies must allocate their communications budget over eight major modes of communications…Companies must consider several factors in developing their communications mix: type of product market, buyer-readiness stage and product-life-cycle stage.” For details, see Marketing Communications Budgeting, pages 553-557.
3. Measure communications’ results: “Senior managers want to know the outcomes and revenues resulting from their investments.” For details, see, Marketing Communications Budgeting, page 557.

None of the recommendations, above, are practicable.  They can not be scaled to the S of SG&A.