Truffle tests: comparing balances after and before and operation

Question

I am doing a test that checks if a reward can't be claimed as soon as the bought is done but 60 seconds later. Banaces are not precise and I am going crazy due to BN and comparision units. this is the test code:

it('Should not let user3 accounts[12] to claim rewards just after buy, but after one day: # claims must update, balance should be OK', async() => {
        const busd = await BUSD.deployed();
        const nftMinter = await NFTMinter.deployed();
        const busdPreBalance = await busd.balanceOf(accounts[12]);
        const op1 = await nftMinter.getMyTokens({from: accounts[12]});
        const nftValue = op1[0].value;
        const expectedReward = op1[0].rewardPerInterval ;

        await catchNotEnoughTimePassed(nftMinter.claimReward({from: accounts[12]}));

        await increase(60);

        await nftMinter.claimReward({from: accounts[12]});
        const busdPostBalance = await busd.balanceOf(accounts[12]);
        const busdExpectedBal = busdPreBalance + expectedReward;
        // actual , expected
        assert.equal(busdPostBalance.toString(), busdExpectedBal, "reward quant is not ok");
    })

And this is the error it throws on last comparision:

 1) Contract: NFTMinter
       Should not let user3 accounts[12] to claim rewards just after buy, but after one day: # claims must update, balance should be OK:

      reward quant is not ok
      + expected - actual

      -992028027681660899653976
      +9920249134948096885813123460207612456747404

can you help please? I'll be grateful!

edit: My real problem is that I can't find the way to multiply an uint256 value from an smart contract, store it on a variable, and multiply itself for 0.9.

const op1 = await nftMinter.getMyTokens({from: accounts[12]});
const expectedReward = op1[0].rewardPerInterval * 0.9;