In this approach; I like to err on the safe side. Send more than enough to that wallet. Then after you have successfully sent in 1 ETH to the ICO contract, you can send the remaining ETH back to your original (cold?) wallet.
Do you spend a little bit more on gas this way? Yes, but the amount spent if negligible given pure ETH transactions are relatively cheap given there is no time constraints or EVM gas storms.
A pro to this method is that you can be sure you have enough funds to cover any unexpected hikes in gas prices to ensure you get into the ICO. Some ICO's, as you may know, can sell out within minutes. Best not to waste time and miss out if by trying to save tiny fractions of ETH.
- Gas Units consumed by a simple ETH transfer: 21000 gas units
- Gas Price: 0.00000001 or 10 GWei
- Total Gas Fee: 0.00021 ether
Current suggested gas price (https://ethgasstation.info/): 2.2 GWei
In short; I recommend sending more than enough and spending the extra 0.00021 ETH to send the funds back to your original wallet. Or leave the tiny amount of ETH in that wallet to cover the gas costs of transferring your tokens to another wallet/contract.
I cannot tell you exactly how much ETH you need to send to your investing wallet. Each ICO is unique, or potentially can be. With this, gas costs vary from ICO to ICO. The only way to know for sure is to monitor other transactions going into that smart contract and possibly what MetaMask (if using MetaMask) calculates for you before signing and submitting your transaction to the mainnet.
Very direct: 1.05 ETH should suffice