This could not be further from the truth. D-Limonene is a solvent. Inhaling too large % quantities is without question harmful to your lungs. Please learn more about this before proceeding with your current assumptions.
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"Health Hazard
SYMPTOMS: Symptoms of exposure to this compound may include irritation and sensitization of the skin. It may also cause eye irritation and damage. Ingestion of large doses may lead to albuminuria and hematuria. This type of compound irritates all tissues intensely and may cause circulatory collapse. Ingestion of this type of compound may cause abdominal burning, nausea, vomiting, diarrhea, dysuria, hematuria, unconsciousness, shallow respiration, and convulsions. Inhalation of this type of compound may cause dizziness, rapid and shallow breathing, tachycardia, bronchial irritation, unconsciousness and convulsions. Anuria, pulmonary edema and bronchia pneumonia may complicate recovery after either type of exposure.
ACUTE/CHRONIC HAZARDS: This compound is a skin irritant and sensitizer. It is also an eye irritant. It may be harmful by ingestion, inhalation or skin absorption. When heated to decomposition it emits acrid smoke and toxic fumes of carbon monoxide and carbon dioxide. (NTP, 1992)"
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http://cameochemicals.noaa.gov/chemical/20568
In stark contrast, medicinal/food grade propolyne glycol, vegetable glycerin, and polyethylene glycol are not proven harmful in quantities/temperatures used for vaping.
**EDIT:
But to answer your original question, you should look at terpene profiles in cannabis strains as a guide. If you believe 5% is what's being used in that prepackaged brand (sounds high for pure limonene to me), then that would be only 1.6 drops in one gram of concentrate.
Anyway, regardless adding too much limonene makes your juice really harsh on your throat. The taste also becomes nasty bitter, like concentrated orange peel. It's very unpleasant.
I don't think you can dissolve concentrate in just pure limonene at any reasonable ratio to create a usable end product. Unless maybe if it was already really runny oil, the limonene needs an additional carrier to help dilute the concentrate.