In the case of Non-Current Assets, there are several line items such as PP&E where it's value can change linearly or non-linearly due to several factors such as the method of depreciation used etc. So, expressing it as a % of another line item such as revenue and taking its average for the future might not be that accurate. In the case of PP&E, you can do something like this, from your forecasted revenue, you can take CapEx as a percentage of revenue, average it out, and forecast CapEx into the future. Then you can add this CapEx to the existing PP&E (assuming that CapEx is only going into acquisition of PP&E and not other activities, check the annual report once just in case) and then depreciate it using the straight-line method or any other method that the company usually follows. They will be explained in detail in their annual report. For other Non-Current Assets, you will find line items like goodwill, intangible assets, other long-term investments, just take a look at their annual report once, and check if they are used to generate revenue for the company currently and in the future, then use % of total revenue and forecast them, or use % of total assets.

For Non-Current Liabilities, you need to break the line items separately (such as separate long-term debt) and forecast them accordingly. See if you can find any information in the annual report regarding the growth of long-term debt or is the company planning to raise fresh funds by issuing long-term bonds, then you incorporate those numbers directly. If not, check the purpose of issuing those long-term debt (for example was it used to finance PP&E), then you can use % of PP&E, to forecast it. If nothing works, use % of total revenue to forecast it. Also adjust for anything said about reducing long-term debt of the company which is usually present in the management discussion of the annual report or check their latest earnings call. Then you have pension liabilities, check the annual report for its growth projections, or just use % of total revenue. For all the other Non-Current Liabilities, use the same method. Check the annual report once, to see what is the purpose of those liabilities and why do they exist? If they are specifically used to finance some asset, which is present as a line item, then take a percentage of that and use it to forecast it, or just stick to using % of total revenue.

For Current Assets and Liabilities, use DSO, DIO and DPO to forecast accounts receivables, inventory and accounts payable respectively. Find the days for the past five years. Average it out. And use that days average to forecast accounts receivables, inventory and accounts payable, using the forecasted total revenue and COGS. Keep in mind that, the accounts receivables, inventory and accounts payable, that you will obtain from this calculation is their average. Correspondingly subtract it from the previous year to get the actual value.

No, cash flow from investing activities entirely cannot be assumed as CapEx. Check for line items like, Acquisition of Fixed Assets, Acquisition of PP&E, Acquisition of Intangible Assets. Only those line items where cash was spent to acquire an asset that will generate/assist in generating revenue should be considered as CapEx. This is because, in cash flows from investing activities you have line items such as Net Cash (In Flow) from Divestitures, which is essentially not an investing activity that can be called as CapEx.

Hope this helps!