how to calculator solana fee without rpc?

[BillInUK]

i send an transaction to trasfer my spl token,in the following code i simulate my transaction to get the comsumed units,and GetFeeForMessage to calculate the transaction fee.

func TestEstimateMultiTransferFee(t *testing.T) {
	fromPrivateKey, _ := solana.PrivateKeyFromSolanaKeygenFile("/Users/mac/.config/solana/priv_alice.json")
	fromNativeAccount := fromPrivateKey.PublicKey()
	toNativeAccounts := []solana.PublicKey{
		solana.MPK("27htRMGeQ4HV32SPHsJrpndZn1zwmF2kiPqmABcHgehx"),
	}
	tokenMintAccount := solana.MPK(TokenMintAddress)
	var amount uint64 = 1000
	var decimals uint8 = 3
	priorityFee, err := querySOLPriorityFee()
	if err != nil {
		panic(err)
	}
	var transferInsts []solana.Instruction
	for index, _ := range toNativeAccounts {
		fromTokenAccount, _, _ := solana.FindAssociatedTokenAddress(fromNativeAccount, tokenMintAccount)
		toTokenAccount, _, _ := solana.FindAssociatedTokenAddress(toNativeAccounts[index], tokenMintAccount)
		inst := token.NewTransferCheckedInstructionBuilder().
			SetAmount(amount).
			SetDecimals(decimals).
			SetSourceAccount(fromTokenAccount).
			SetMintAccount(tokenMintAccount).
			SetDestinationAccount(toTokenAccount).
			SetOwnerAccount(fromNativeAccount).
			Build()
		transferInsts = append(transferInsts, inst)
	}

	// 构造1笔交易用来评估手续费
	computePrice := priorityFee.PerTransaction.Medium
	computeBudgetPriceInst := computebudget.NewSetComputeUnitPriceInstructionBuilder().SetMicroLamports(computePrice).Build()
	var simulateInstructions []solana.Instruction
	simulateInstructions = append(simulateInstructions, computeBudgetPriceInst)
	simulateInstructions = append(simulateInstructions, transferInsts...)

	recent, err := rpcClient.GetLatestBlockhash(context.Background(), rpc.CommitmentFinalized)
	if err != nil {
		panic(err)
	}

	// simulate执行交易
	simulateSolTx, err := solana.NewTransaction(simulateInstructions, recent.Value.Blockhash, solana.TransactionPayer(fromNativeAccount))
	if err != nil {
		panic(err)
	}
	msgBin, _ := simulateSolTx.Message.MarshalBinary()
	fromSign, _ := fromPrivateKey.Sign(msgBin)
	simulateSolTx.Signatures = append(simulateSolTx.Signatures, fromSign)
	simulateResult, err := rpcClient.SimulateTransactionWithOpts(context.Background(), simulateSolTx, &rpc.SimulateTransactionOpts{
		ReplaceRecentBlockhash: true,
	})
	if err != nil {
		panic(err)
	}
	computeUnits := uint32(*simulateResult.Value.UnitsConsumed)
	computeBudgetLimitInst := computebudget.NewSetComputeUnitLimitInstructionBuilder().SetUnits(computeUnits).Build()
	simulateInstructions = append(simulateInstructions, computeBudgetLimitInst)

	fmt.Printf("computeUnitPrice %d\n", computePrice)
	fmt.Printf("computeUnitLimit %d\n", computeUnits)

	// 获取手续费
	base64EncodeMsg := base64.StdEncoding.EncodeToString(msgBin)
	fee, err := rpcClient.GetFeeForMessage(context.Background(), base64EncodeMsg, rpc.CommitmentFinalized)
	if err != nil {
		panic(err)
	}
	if fee == nil || *fee.Value == 0 {
		panic("estimate fee returns 0")
	}
	fmt.Printf("estimate fee %d\n", *fee.Value)

	txId, err := rpcClient.SendTransactionWithOpts(context.Background(), simulateSolTx, rpc.TransactionOpts{SkipPreflight: true})
	if err != nil {
		panic(err)
	} else {
		fmt.Printf("%s\n", txId.String())
		fmt.Printf("https://explorer.solana.com/tx/%s?cluster=devnet\n", txId.String())
	}

}

the output of the code is :

computeUnitPrice 50003
computeUnitLimit 6322
estimate fee 15001
2jUhRghTdtZ3pwszNK868njTWdke3V7zPikbYHzbNqMxdnjGR1PPbGJSomek7PMcnVKKmz3UxMJJdVEFPijaSvhG
https://explorer.solana.com/tx/2jUhRghTdtZ3pwszNK868njTWdke3V7zPikbYHzbNqMxdnjGR1PPbGJSomek7PMcnVKKmz3UxMJJdVEFPijaSvhG?cluster=devnet
--- PASS: TestEstimateMultiTransferFee (2.38s)
PASS

I followed Solana's official method for calculating transaction fees, but I am unable to get a result of 15001. Could you please explain how the fee of 15001 was calculated?