cpu_frac and net_frac calculation

Start at the rentResources entry point and trace powerup.cpu.frac_by_ms, powerup.net.frac_by_kb, and the POWER_UP action parameters. Reproduce the cases where either fraction is zero, then verify the resource calculation and action behavior for CPU-only, NET-only, and combined rentals.

i'm trying to rent cpu and net resources, but find out very strange behavior. when im trying to buy net, so my cpu_frac is 0, it only purchase cpu resource, i discovered the other way is also true. in my case, how should i adjust my code to get rid of this behavior of power up model and rent resources correctly?

briefly, i want to know how to calculate cpu and net frac correctly, thanks!

  async rentResources(params: RentResourcesDto): Promise<AccountResource> {
    const { payer, reciver, cpuAmountMs, netAmountKb } = params;

    const apiClient = getClient().apiClient;
    const eosResources = new Resources({
      api: apiClient,
    });

    const [powerup, rexState, ramState, sampleUsage] =
      await Promise.all([
        eosResources.v1.powerup.get_state(),
        eosResources.v1.rex.get_state(),
        eosResources.v1.ram.get_state(),
        eosResources.getSampledUsage(),
      ]);

    const cpuPrice = powerup.cpu.price_per_ms(sampleUsage, cpuAmountMs);
    const netPrice = powerup.net.price_per_kb(sampleUsage, netAmountKb);

    const totalCost = cpuPrice + netPrice;

    const days = powerup.powerup_days;

    // weird problematic part?
    const cpu_frac = powerup.cpu.frac_by_ms(sampleUsage, cpuAmountMs);
    const net_frac = powerup.net.frac_by_kb(sampleUsage, netAmountKb);

    const powerUpAction = contract.action(
      EOS_ACTIONS.POWER_UP,
      {
        payer,
        receiver: reciver,
        days,
        net_frac: net_frac,
        cpu_frac: cpu_frac,
        max_payment: `${totalCost.toFixed(NATIVE_DECIMALS)} ${EOS_SYMBOL}`,
      },
      {
        authorization: [
          {
            actor: payer,
            permission: ACTIVE_PERMISSION,
          },
        ],
      },
    );
...