Amputation ratings are set by VASRD body-system tables and are almost always evaluated highly for the limb itself, but your combined rating depends on residual limb condition, phantom pain, and any secondary conditions — most veterans under-claim the secondary conditions, not the amputation itself.
The VA rates amputation under 38 CFR Part 4, the VASRD — a below-elbow or below-knee amputation is rated differently from above-joint loss, and the exact level matters more than most first-time claimants expect. DRAFT NOTE: this needs a full pass on the current rating tables and percentages, plus a real reviewer (someone with lived experience or VA claims expertise) attached, before this is published.